Prevent sharing a diagram with yourself

RLS is the real guard (diagram_collaborators_insert/update now reject
user_id = the diagram's owner, regardless of who's performing the write —
covers a Super Admin acting on someone else's diagram too, not just the
normal owner path); the client-side check in
SupabaseDiagramCollaboratorRepository.add is just there to surface a
friendly message instead of the raw 42501.

Verified: tsc -b and oxlint clean; supabase db reset + 53/53 pgTAP tests
pass (1 new test).

src/components/sharing/DiagramSharingModal.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useState } from 'react'
 import type { CollaboratorPermission } from '../../data/DiagramCollaboratorRepository'
-import { UnknownUsernameError } from '../../data/DiagramCollaboratorRepository'
+import { SelfCollaboratorError, UnknownUsernameError } from '../../data/DiagramCollaboratorRepository'
 import { useDiagramCollaboratorStore } from '../../state/diagramCollaboratorStore'
 import { useProjectStore } from '../../state/projectStore'
 import Modal from '../common/Modal'
@@ -36,7 +36,11 @@ export default function DiagramSharingModal({ onClose }: { onClose: () => void }
       await add(diagramId, trimmed, permission)
       setUsername('')
     } catch (err) {
-      setError(err instanceof UnknownUsernameError ? err.message : 'Could not add that person. Try again.')
+      setError(
+        err instanceof UnknownUsernameError || err instanceof SelfCollaboratorError
+          ? err.message
+          : 'Could not add that person. Try again.',
+      )
     } finally {
       setBusy(false)
     }