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.

frontend/src/types.ts

@@ -0,0 +1,56 @@
+export type BBox = [number, number, number, number] // x, y, w, h, normalized 0-1
+
+export interface Item {
+  id: string
+  label: string
+  price: number
+  bbox: BBox | null
+}
+
+export interface Receipt {
+  id: string
+  items: Item[]
+  store_name: string | null
+  date: string // YYYY-MM-DD; extracted, falls back to today, user-editable
+  status: 'extracted' | 'grouped' | 'done'
+  created_at: string
+}
+
+export interface Member {
+  id: string
+  name: string
+}
+
+export interface CospendProject {
+  id: string
+  name: string
+}
+
+export interface Group {
+  id: string
+  receipt_id: string
+  name: string
+  cospend_project_id: string
+  payer_member_id: string
+  member_ids: string[]
+  item_ids: string[]
+  status: 'pending' | 'submitted' | 'failed'
+  share_url: string | null
+  cospend_bill_id: string | null
+  error: string | null
+}
+
+export interface SubmitResult {
+  share_url: string
+  amount: number
+  bill_id: number
+}
+
+/** A group draft in progress client-side, before it's persisted via POST /groups. */
+export interface GroupDraft {
+  localId: string
+  name: string
+  payerMemberId: string | null
+  memberIds: string[]
+  itemIds: string[]
+}