Initial scaffold: Flask/SQLite backend, React/Vite PWA frontend

Backend: receipt upload -> LLM vision extraction (OpenAI-compatible,
provider-agnostic), item review/edit, per-group splitting against
Cospend projects/members, highlight+upload via WebDAV, public share
link, bill creation via Cospend's OCS API (verified against real
source, not just doc summaries).

Frontend: capture -> review -> group -> summary flow as an installable
PWA.

install.sh / run.sh (venv + npm, tmux session) instead of Docker, per
the ~/Projects/gain pattern.

backend/app/highlight.py

@@ -0,0 +1,32 @@
+"""Draws highlight boxes over selected items on the original receipt image."""
+
+from __future__ import annotations
+
+import io
+
+from PIL import Image, ImageDraw
+
+_BOX_COLOR = (255, 210, 0)  # translucent yellow highlighter look
+_BOX_WIDTH = 4
+
+
+def highlight_items(image_bytes: bytes, bboxes: list[list[float]]) -> bytes:
+    """`bboxes` are normalized [x, y, w, h] (0-1). Returns JPEG bytes."""
+    base = Image.open(io.BytesIO(image_bytes)).convert("RGBA")
+    overlay = Image.new("RGBA", base.size, (0, 0, 0, 0))
+    draw = ImageDraw.Draw(overlay)
+    w, h = base.size
+
+    for bbox in bboxes:
+        if not bbox or len(bbox) != 4:
+            continue
+        x, y, bw, bh = bbox
+        left, top = x * w, y * h
+        right, bottom = (x + bw) * w, (y + bh) * h
+        draw.rectangle([left, top, right, bottom], fill=(*_BOX_COLOR, 70))
+        draw.rectangle([left, top, right, bottom], outline=(*_BOX_COLOR, 255), width=_BOX_WIDTH)
+
+    combined = Image.alpha_composite(base, overlay).convert("RGB")
+    out = io.BytesIO()
+    combined.save(out, format="JPEG", quality=90)
+    return out.getvalue()