feat(errors): per-account error modal with persisted error list

Accounts finishing done_with_errors showed only a count and a single
last_error. This adds a modal listing every concrete error of the
account's most recent run.

- migration 0005: account_errors table (kind folder|message|account,
  folder, message_ref, error, created_at; ON DELETE CASCADE; indexed)
- store: AddAccountError / ClearAccountErrors / ListAccountErrors
- copy: OnError callback captures per-message error text (previously
  only counted), with a "UID N: subject" reference
- orchestrator: clear errors at run start; persist folder/message/
  account errors; cap 500 rows/account/run with a suppressed-note row
- api: GET /api/tasks/{id}/accounts/{accountId}/errors
- web: AccountErrorsModal, clickable ERRORS count, api + styles

Verified: migration applies on Postgres 18; store add/list/clear and
cascade tests pass against real pg; backend build/vet/test green; web
tsc+vite build and oxlint clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U9Eq4JtWjyNTv5qat3B3mM

internal/store/store_test.go

@@ -17,7 +17,7 @@ func testStore(t *testing.T) *Store {
 	}
 	t.Cleanup(func() {
 		s.Pool.Exec(context.Background(),
-			`TRUNCATE endpoints, tasks, accounts, runs, migrated_messages RESTART IDENTITY CASCADE`)
+			`TRUNCATE endpoints, tasks, accounts, runs, migrated_messages, account_errors RESTART IDENTITY CASCADE`)
 		s.Pool.Close()
 	})
 	return s