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/App.tsx

@@ -1,5 +1,28 @@
+import type { Session } from '@supabase/supabase-js'
+import { useEffect, useState } from 'react'
+import LoginScreen from './components/auth/LoginScreen'
 import AppShell from './components/layout/AppShell'
+import { supabase } from './data/supabaseClient'
 
 export default function App() {
+  // undefined = still checking for an existing session; null = signed out.
+  const [session, setSession] = useState<Session | null | undefined>(undefined)
+
+  useEffect(() => {
+    supabase.auth.getSession().then(({ data }) => setSession(data.session))
+    const {
+      data: { subscription },
+    } = supabase.auth.onAuthStateChange((_event, session) => setSession(session))
+    return () => subscription.unsubscribe()
+  }, [])
+
+  if (session === undefined) {
+    return <div className="flex h-screen items-center justify-center text-sm text-slate-400">Loading…</div>
+  }
+
+  if (!session) {
+    return <LoginScreen />
+  }
+
   return <AppShell />
 }