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/TopBar.tsx

@@ -1,22 +1,15 @@
-import { useRef } from 'react'
+import { useRef, useState } from 'react'
 import { downloadProjectFile, readProjectFile, ProjectImportError } from '../../data/exportImport'
 import { supabase } from '../../data/supabaseClient'
 import { useProjectStore } from '../../state/projectStore'
+import DiagramManagerModal from './DiagramManagerModal'
 
 export default function TopBar() {
   const project = useProjectStore((s) => s.project)
   const renameProject = useProjectStore((s) => s.renameProject)
-  const newProject = useProjectStore((s) => s.newProject)
   const importProject = useProjectStore((s) => s.importProject)
   const fileInputRef = useRef<HTMLInputElement>(null)
-
-  const handleNewProject = () => {
-    if (project.devices.length > 0 || project.connections.length > 0) {
-      const confirmed = window.confirm('Start a new project? Unsaved changes to the current one will be lost (unless exported).')
-      if (!confirmed) return
-    }
-    newProject()
-  }
+  const [diagramManagerOpen, setDiagramManagerOpen] = useState(false)
 
   const handleExport = () => downloadProjectFile(project)
 
@@ -28,7 +21,7 @@ export default function TopBar() {
     if (!file) return
     try {
       const imported = await readProjectFile(file)
-      importProject(imported)
+      await importProject(imported)
     } catch (err) {
       const message = err instanceof ProjectImportError ? err.message : 'Could not read that file.'
       window.alert(`Import failed: ${message}`)
@@ -46,8 +39,11 @@ export default function TopBar() {
         />
       </div>
       <div className="flex items-center gap-1.5">
-        <button onClick={handleNewProject} className="rounded px-2.5 py-1.5 text-xs text-slate-600 hover:bg-slate-100">
-          New
+        <button
+          onClick={() => setDiagramManagerOpen(true)}
+          className="rounded px-2.5 py-1.5 text-xs text-slate-600 hover:bg-slate-100"
+        >
+          Diagrams
         </button>
         <button onClick={handleImportClick} className="rounded px-2.5 py-1.5 text-xs text-slate-600 hover:bg-slate-100">
           Import
@@ -66,6 +62,7 @@ export default function TopBar() {
           Sign out
         </button>
       </div>
+      {diagramManagerOpen && <DiagramManagerModal onClose={() => setDiagramManagerOpen(false)} />}
     </header>
   )
 }