Add site-wide announcements, account migration, and profile self-service

Announcements: a Super Admin (or an Admin individually flagged via
profiles.can_post_announcements) can post/retire a site-wide banner.
Account migration: a Super Admin can move a locked-out user's diagrams,
private catalog entries, and submissions to another account, with a
migration-history log; ProfileModal adds the self-service half (link a new
Google identity via Supabase manual linking, then unlink the old one,
while signed in as the account being migrated). Also reworks the top bar's
flat button row into grouped dropdown menus (Diagram / Admin / Account) now
that there are enough entries to need it.

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

src/state/authStore.ts

@@ -8,6 +8,10 @@ interface AuthStoreState {
   userId: string | null
   username: string | null
   role: UserRole | null
+  /** Lets a specific Admin post/retire site-wide announcements without the
+   *  whole Admin role gaining that ability — a Super Admin can always post
+   *  regardless of this flag (see can_manage_announcements() in the DB). */
+  canPostAnnouncements: boolean
   isLoaded: boolean
 
   /** One-time fetch of your own profile — nothing else in the app changes
@@ -22,6 +26,7 @@ export const useAuthStore = create<AuthStoreState>((set) => ({
   userId: null,
   username: null,
   role: null,
+  canPostAnnouncements: false,
   isLoaded: false,
 
   load: async () => {
@@ -32,11 +37,16 @@ export const useAuthStore = create<AuthStoreState>((set) => ({
       set({ isLoaded: true })
       return
     }
-    const { data } = await supabase.from('profiles').select('username, role').eq('id', user.id).single()
+    const { data } = await supabase
+      .from('profiles')
+      .select('username, role, can_post_announcements')
+      .eq('id', user.id)
+      .single()
     set({
       userId: user.id,
       username: data?.username ?? null,
       role: (data?.role as UserRole | undefined) ?? null,
+      canPostAnnouncements: data?.can_post_announcements ?? false,
       isLoaded: true,
     })
   },