Add diagram sharing/collaborators, version history, and view-only lockdown

Per organized-ideas.md §8. Backend tables/RLS (diagrams, diagram_collaborators,
diagram_snapshots) already existed from an earlier phase — this is the
frontend for them, plus two small backend additions.

Backend (supabase/migrations/20260913000000_diagram_sharing.sql):
- find_user_id_by_username(text): lets any authenticated user resolve a
  username to an id for "share with @username" — unlike general profile
  browsing (blocked by profiles_select_self_or_super_admin), a username is
  meant to be a shareable handle, so this is deliberately not gated.
- diagram_collaborator_usernames / diagram_snapshot_saved_by_usernames:
  same pattern as the admin-review-queue phase's submitter-username
  lookup — batched per diagram, gated to "can you see this diagram at all"
  (reusing diagrams_select's own helper functions).
- prune_diagram_snapshots trigger: keeps the 50 most recent snapshots per
  diagram, enforced at write time rather than a scheduled job (diagram_
  snapshots has no update/delete policy for regular users at all).
- 14 new pgTAP tests (52/52 total).

Frontend:
- DiagramCollaboratorRepository/store + DiagramSharingModal: add/remove
  collaborators by username, per-person view/edit permission, owner-only
  controls.
- DiagramSnapshotRepository/store + VersionHistoryModal (its own top-bar
  button, not nested under Share — moved there after review): periodic
  checkpoints (one per 5 min of active editing) written as a side effect
  of normal saves, list + restore.
- Restore's duplicate-snapshot problem: repeatedly jumping between old
  versions without editing in between was writing a near-duplicate safety
  snapshot on every jump. Fixed by having projectStore track which
  snapshot the diagram was last restored from and its updatedAt at that
  moment (touch() always advances updatedAt on a genuine edit) — a restore
  skips the safety snapshot when nothing has changed since the last one,
  and the tracking clears on any real edit so in-progress work stays
  protected.
- DiagramRepository gains getAccess() (owner id + your own permission for
  the open diagram) — surfaced in projectStore as `access`.
- View-only enforcement: FlowCanvas disables drag/connect/drop
  (nodesDraggable/nodesConnectable + guarded handlers), DeviceInspector/
  ConnectionInspector wrap their controls in a disabled <fieldset>,
  DevicePalette disables adding devices to the canvas, TopBar disables the
  rename field, and a ViewOnlyBanner makes the restriction visible instead
  of leaving a collaborator to discover it as controls that just don't
  work. Autosave itself also refuses to write for a view-only user, as a
  backstop behind the UI-level lockdown.

Verified: tsc -b and oxlint clean; supabase db reset + 52/52 pgTAP tests
pass; confirmed find_user_id_by_username works through the real REST API
via a live curl call (signup, confirm, resolve). Manually tested two-
account sharing (view vs. edit), restoring history, and the duplicate-
snapshot fix.

src/data/DiagramRepository.ts

@@ -19,6 +19,16 @@ export interface DiagramSummary {
  * user-facing copy still say "Project" — that's a separate, larger mechanical rename
  * across the whole app, tracked as its own task rather than bundled into this one.
  */
+/** Your relationship to a diagram — who owns it, and what you personally
+ *  can do with it (organized-ideas.md §8's per-collaborator view/edit
+ *  permissions). Drives the frontend's own view-only lockdown, since RLS
+ *  silently no-ops a blocked write rather than erroring — the UI needs to
+ *  know *before* the user tries, not just fail quietly after. */
+export interface DiagramAccess {
+  ownerId: string
+  myPermission: 'owner' | 'edit' | 'view'
+}
+
 export interface DiagramRepository {
   /** Every diagram visible to the current user, most recently updated first. */
   list(): Promise<DiagramSummary[]>
@@ -28,4 +38,6 @@ export interface DiagramRepository {
   save(project: Project): Promise<void>
   /** Permanently delete one diagram by id. */
   deleteById(id: string): Promise<void>
+  /** Who owns this diagram and what the current user can do with it. */
+  getAccess(id: string): Promise<DiagramAccess | null>
 }