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/data/supabaseClient.ts

@@ -0,0 +1,13 @@
+import { createClient } from '@supabase/supabase-js'
+
+const url = import.meta.env.VITE_SUPABASE_URL
+const anonKey = import.meta.env.VITE_SUPABASE_ANON_KEY
+
+if (!url || !anonKey) {
+  throw new Error(
+    'Missing VITE_SUPABASE_URL / VITE_SUPABASE_ANON_KEY. Copy .env.example to .env.local and fill them in ' +
+      '(run `npx supabase start` then `npx supabase status` for local values).',
+  )
+}
+
+export const supabase = createClient(url, anonKey)