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.

run.sh

@@ -0,0 +1,37 @@
+#!/bin/bash
+set -e
+
+echo "[*] Starting wgBill Local Native Environment..."
+
+SESSION="wgbill"
+SESSIONEXISTS=$(tmux list-sessions 2>/dev/null | grep $SESSION || true)
+
+# Only create tmux session if it doesn't exist
+if [ "$SESSIONEXISTS" = "" ]
+then
+    echo "[.] Starting new tmux session..."
+    tmux new-session -d -s $SESSION
+
+    sleep 3
+
+    # Backend (native)
+    tmux new-window -t $SESSION -n 'backend'
+    tmux send-keys -t "$SESSION:backend" 'cd ./backend && source .venv/bin/activate && python run.py' C-m
+
+    sleep 5
+
+    # Frontend (native)
+    tmux new-window -t $SESSION -n 'frontend'
+    tmux send-keys -t "$SESSION:frontend" 'cd ./frontend && npm run dev' C-m
+
+    tmux select-layout -t $SESSION tiled
+
+else
+    echo "[i] Found existing session."
+    sleep 1
+fi
+
+echo "Attaching to tmux session..."
+sleep 1
+
+tmux attach-session -t $SESSION