Replace the plain-text shell dropdown/list with an icon picker

Both places the app asks for a shell (the Add Strike dialog's
Adw.ComboRow, and the firing panel's per-target shell popover) used to
show just the bare enum name in a plain list, nothing conveying what
the shell actually is. New icons.py builds a shared MenuButton +
popover from the game's own shell icon set (assets/icons/shells/,
already named to match Shell.name exactly): each row shows the shell's
icon, name, description, and blast radius, and the button face updates
to match whichever one gets picked.

Verified via a GTK smoke test that the button and its popover build and
open without crashing.

src/fenigma/app.py

@@ -24,7 +24,7 @@ gi.require_version("Gdk", "4.0")
 from gi.repository import Adw, Gdk, Gio, GLib, Gtk  # noqa: E402
 from PIL import Image  # noqa: E402
 
-from . import ballistics, ocr, solver  # noqa: E402
+from . import ballistics, icons, ocr, solver  # noqa: E402
 from .coord_dialog import CoordDialog  # noqa: E402
 from .firing_panel import FiringPanel  # noqa: E402
 from .grid_widget import GridCanvas  # noqa: E402
@@ -972,12 +972,14 @@ class MainWindow(Adw.ApplicationWindow):
 
         outer = Gtk.Box(orientation=Gtk.Orientation.VERTICAL, spacing=16,
                          margin_top=16, margin_bottom=16, margin_start=16, margin_end=16)
-        group = Adw.PreferencesGroup(title="Shell (blast radius)")
-        shells = list(Shell)
-        shell_row = Adw.ComboRow(title="Shell", model=Gtk.StringList.new([s.name for s in shells]))
-        shell_row.set_selected(shells.index(Shell.HCHE))
-        group.add(shell_row)
-        outer.append(group)
+        label = Gtk.Label(label="Shell (blast radius)", xalign=0)
+        label.add_css_class("heading")
+        outer.append(label)
+
+        chosen = [Shell.HCHE]
+        shell_btn = icons.build_shell_button(chosen[0], lambda s: chosen.__setitem__(0, s))
+        shell_btn.set_halign(Gtk.Align.START)
+        outer.append(shell_btn)
 
         next_btn = Gtk.Button(label="Next: click the map to place it")
         next_btn.add_css_class("suggested-action")
@@ -985,7 +987,7 @@ class MainWindow(Adw.ApplicationWindow):
         next_btn.set_halign(Gtk.Align.CENTER)
 
         def on_next(_b):
-            chosen_shell = shells[shell_row.get_selected()]
+            chosen_shell = chosen[0]
             dialog.close()
             self.canvas.start_placement(
                 lambda coord: self._add_strike_at(coord, chosen_shell),

src/fenigma/firing_panel.py

@@ -29,7 +29,7 @@ gi.require_version("Gtk", "4.0")
 gi.require_version("Gdk", "4.0")
 from gi.repository import Gdk, GObject, Gtk  # noqa: E402
 
-from . import ballistics
+from . import ballistics, icons
 from .models import Board, Target, TargetType
 from .shells import Shell
 
@@ -381,21 +381,8 @@ class FiringPanel(Gtk.Box):
                            charges: int, elev_value_label: Gtk.Label) -> Gtk.Widget:
         row = Gtk.Box(orientation=Gtk.Orientation.HORIZONTAL, spacing=4)
 
-        shell_btn = Gtk.MenuButton(label=target.effective_shell.name)
-        shell_btn.add_css_class("flat")
+        shell_btn = icons.build_shell_button(target.effective_shell, lambda s: self._pick_shell(target, s))
         shell_btn.set_tooltip_text("Change shell (blast radius)")
-        popover = Gtk.Popover()
-        shell_list = Gtk.Box(orientation=Gtk.Orientation.VERTICAL, spacing=2,
-                              margin_top=6, margin_bottom=6, margin_start=6, margin_end=6)
-        for s in Shell:
-            radius = f"{s.blast_radius_km}km" if s.blast_radius_km is not None else "unknown radius"
-            btn = Gtk.Button(label=f"{s.name}: {s.description} ({radius})")
-            btn.add_css_class("flat")
-            btn.get_child().set_xalign(0)
-            btn.connect("clicked", lambda _b, s=s: self._pick_shell(target, s, popover))
-            shell_list.append(btn)
-        popover.set_child(shell_list)
-        shell_btn.set_popover(popover)
         row.append(shell_btn)
 
         segments: list[Gtk.Button] = []
@@ -437,7 +424,6 @@ class FiringPanel(Gtk.Box):
         target.alive = not target.alive
         self.on_change()
 
-    def _pick_shell(self, target: Target, shell: Shell, popover: Gtk.Popover) -> None:
+    def _pick_shell(self, target: Target, shell: Shell) -> None:
         target.shell = shell
-        popover.popdown()
         self.on_change()

src/fenigma/icons.py

@@ -0,0 +1,98 @@
+"""Bundled game-icon lookup and the shared shell picker built from them.
+
+Icon files live in the repo's assets/icons/ (see its README.md for where
+they came from and which source-filename typos got corrected on copy),
+outside the src/ package, resolved relative to this file rather than the
+process's cwd so it works no matter where the app was launched from.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import gi
+
+gi.require_version("GdkPixbuf", "2.0")
+gi.require_version("Gtk", "4.0")
+from gi.repository import GdkPixbuf, Gtk  # noqa: E402
+
+from .shells import Shell
+
+_ICONS_DIR = Path(__file__).resolve().parent.parent.parent / "assets" / "icons"
+
+
+def shell_icon_path(shell_name: str) -> Path:
+    return _ICONS_DIR / "shells" / f"{shell_name}.png"
+
+
+def shell_icon_image(shell_name: str, width: int = 64) -> Gtk.Image:
+    """A Gtk.Image for a Shell enum member's icon, scaled to `width` px
+    wide (the source art is a fixed 2:1 rectangle, height follows).
+    Falls back to a generic missing-image icon rather than raising, a
+    gap in the icon set shouldn't crash the shell picker."""
+    path = shell_icon_path(shell_name)
+    if path.exists():
+        pixbuf = GdkPixbuf.Pixbuf.new_from_file_at_scale(str(path), width, width // 2, False)
+        return Gtk.Image.new_from_pixbuf(pixbuf)
+    image = Gtk.Image.new_from_icon_name("image-missing-symbolic")
+    image.set_pixel_size(width // 2)
+    return image
+
+
+def _shell_row(s: Shell) -> Gtk.Widget:
+    radius = f"{s.blast_radius_km}km blast radius" if s.blast_radius_km is not None else "blast radius unknown"
+    content = Gtk.Box(orientation=Gtk.Orientation.HORIZONTAL, spacing=10)
+    content.append(shell_icon_image(s.name, width=56))
+    label_box = Gtk.Box(orientation=Gtk.Orientation.VERTICAL, spacing=0, hexpand=True)
+    name_label = Gtk.Label(label=s.name, xalign=0)
+    name_label.add_css_class("heading")
+    desc_label = Gtk.Label(label=f"{s.description} · {radius}", xalign=0, wrap=True)
+    desc_label.add_css_class("caption")
+    desc_label.add_css_class("dim-label")
+    label_box.append(name_label)
+    label_box.append(desc_label)
+    content.append(label_box)
+    return content
+
+
+def build_shell_popover(on_pick) -> Gtk.Popover:
+    """Popover listing every Shell with its icon, name, description, and
+    blast radius, replacing a plain text dropdown/list with something
+    that actually shows what each shell looks like. Calls `on_pick(shell)`
+    and closes itself when a row is clicked."""
+    popover = Gtk.Popover()
+    scroller = Gtk.ScrolledWindow(max_content_height=420, propagate_natural_height=True)
+    box = Gtk.Box(orientation=Gtk.Orientation.VERTICAL, spacing=2,
+                  margin_top=6, margin_bottom=6, margin_start=6, margin_end=6)
+    for s in Shell:
+        row = Gtk.Button(child=_shell_row(s))
+        row.add_css_class("flat")
+        row.connect("clicked", lambda _b, s=s: (popover.popdown(), on_pick(s)))
+        box.append(row)
+    scroller.set_child(box)
+    popover.set_child(scroller)
+    return popover
+
+
+def build_shell_button(selected: Shell, on_pick) -> Gtk.MenuButton:
+    """A flat MenuButton showing the currently selected shell's icon and
+    name, opening build_shell_popover() to change it. `on_pick(shell)`
+    fires on selection, after this button's own face has already been
+    updated to match, the caller only needs to react to the new value
+    (persist it, refresh dependents), not maintain the button's display."""
+    btn = Gtk.MenuButton()
+    btn.add_css_class("flat")
+
+    def render(s: Shell) -> None:
+        content = Gtk.Box(orientation=Gtk.Orientation.HORIZONTAL, spacing=6)
+        content.append(shell_icon_image(s.name, width=28))
+        content.append(Gtk.Label(label=s.name))
+        btn.set_child(content)
+
+    def handle_pick(s: Shell) -> None:
+        render(s)
+        on_pick(s)
+
+    render(selected)
+    btn.set_popover(build_shell_popover(handle_pick))
+    return btn