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/components/inspector/ConnectionInspector.tsx

@@ -6,6 +6,8 @@ import { useProjectStore } from '../../state/projectStore'
 export default function ConnectionInspector({ connectionId }: { connectionId: string }) {
   const project = useProjectStore((s) => s.project)
   const catalog = useCatalogStore((s) => s.catalog)
+  const access = useProjectStore((s) => s.access)
+  const canEdit = access?.myPermission !== 'view'
   const updateConnection = useProjectStore((s) => s.updateConnection)
   const removeConnection = useProjectStore((s) => s.removeConnection)
 
@@ -27,7 +29,7 @@ export default function ConnectionInspector({ connectionId }: { connectionId: st
   if (!connection || !sourceDevice || !targetDevice || !sourcePort || !targetPort) return null
 
   return (
-    <div className="space-y-3 p-3">
+    <fieldset disabled={!canEdit} className="m-0 space-y-3 border-0 p-3">
       <div className="rounded border border-slate-200 bg-slate-50 p-2 text-xs">
         <div className="flex items-center justify-between py-0.5">
           <span className="text-slate-500">From</span>
@@ -132,6 +134,6 @@ export default function ConnectionInspector({ connectionId }: { connectionId: st
           Remove cable
         </button>
       </div>
-    </div>
+    </fieldset>
   )
 }