Wire the app to the Supabase backend, replacing localStorage-only

- DiagramRepository (renamed from ProjectRepository, per the
  organized-ideas.md §8 naming decision) now has a Supabase-backed
  implementation as the active repository. LocalStorageDiagramRepository
  stays in the codebase as a reference implementation / fallback, just
  no longer wired in. Only the storage layer's naming changed here --
  the domain type, store, and UI copy still say "Project"; that's a
  separate, larger mechanical rename tracked on its own.
- Minimal email/password auth gate (src/components/auth/LoginScreen.tsx)
  since Supabase RLS requires a real signed-in user to do anything --
  this is NOT the Phase 2 experience (Google SSO, polished signup),
  just enough of the same schema (username + email + password) to make
  the backend foundation usable end to end before that phase exists.
  Respects the hard email-verification gate from config.toml.
- .env.example documents the required VITE_SUPABASE_URL /
  VITE_SUPABASE_ANON_KEY (local dev values, not secrets); .env.local
  has the actual local values and is gitignored.

Verified end-to-end against the local stack: signup creates a
confirmed-pending user, the handle_new_user trigger creates their
profile, sign-in is blocked until confirmed, and a signed-in session
can upsert/read back its own diagram row exactly as the app's
save()/load() do it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017DUU6CnxECCDeqDNYJgr5x

src/components/layout/TopBar.tsx

@@ -1,5 +1,6 @@
 import { useRef } from 'react'
 import { downloadProjectFile, readProjectFile, ProjectImportError } from '../../data/exportImport'
+import { supabase } from '../../data/supabaseClient'
 import { useProjectStore } from '../../state/projectStore'
 
 export default function TopBar() {
@@ -58,6 +59,12 @@ export default function TopBar() {
         >
           Export
         </button>
+        <button
+          onClick={() => supabase.auth.signOut()}
+          className="rounded px-2.5 py-1.5 text-xs text-slate-500 hover:bg-slate-100"
+        >
+          Sign out
+        </button>
       </div>
     </header>
   )