Add multi-diagram management

- DiagramRepository interface redesigned around multiple diagrams:
  list()/loadById()/deleteById() replace the old single-diagram
  load()/clear(). Both implementations updated to match.
- SupabaseDiagramRepository.save() now does an explicit update-or-insert
  instead of a blind upsert, so owner_id is only ever set at creation --
  an upsert would resend it on every save and let whoever saves last
  silently reassign ownership. Not reachable yet (no collaborator UI),
  but a real landmine once diagram sharing (organized-ideas.md §8) lands,
  and cheap to avoid now.
- LocalStorageDiagramRepository now stores diagrams keyed by id (was a
  single fixed key), keeping it a genuine working fallback rather than
  a stale reference implementing an old interface.
- Store: loadInitialDiagram (renamed from loadFromStorage) opens the
  last diagram you had open (tracked in localStorage -- a UI
  preference, not app data), falling back to the most recently updated
  one, falling back to a fresh empty diagram. New actions:
  refreshDiagramList, switchToDiagram, deleteDiagram. newProject and
  importProject now persist immediately (not just via the debounced
  autosave) so a new/imported diagram shows up in the list right away;
  importProject also assigns a fresh id so it can't collide with an
  existing diagram.
- New DiagramManagerModal (list/open/delete/+New), opened from a
  "Diagrams" button in TopBar that replaces the old single-diagram
  "New" button and its now-unnecessary confirmation dialog -- nothing
  is lost by creating a new diagram anymore, since the old one stays
  saved and reachable from the list.

Verified insert/list/update(-preserves-owner)/loadById/delete against
the real local stack; RLS test suite still 23/23 after a fresh reset.

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

src/components/layout/AppShell.tsx

@@ -8,11 +8,11 @@ import TopBar from './TopBar'
 
 export default function AppShell() {
   const isLoaded = useProjectStore((s) => s.isLoaded)
-  const loadFromStorage = useProjectStore((s) => s.loadFromStorage)
+  const loadInitialDiagram = useProjectStore((s) => s.loadInitialDiagram)
 
   useEffect(() => {
-    loadFromStorage()
-  }, [loadFromStorage])
+    loadInitialDiagram()
+  }, [loadInitialDiagram])
 
   if (!isLoaded) {
     return <div className="flex h-screen items-center justify-center text-sm text-slate-400">Loading…</div>