[Epic] Account workspace follow-ups from the #1689 review

Follow-ups from the review of #1689 (consolidate the account features into one `/account` workspace). None of these block that PR; the three blocking items were posted inline on the PR itself. This epic groups the rest so they don't get lost once the workspace lands.

Sub-issues are linked below. Roughly in priority order:

1. Stale "Saved to your account" after sign-out (correctness, small)
2. `FavoritesProvider` ignores auth loading, one frame where a click can duplicate a favorite (correctness, small)
3. Missing `users` row maps to a misleading 409 / silent no-op on the auto-save path (correctness, small)
4. Legacy account routes: three copy-pasted redirect stubs, and `/account/preferences` now 404s (simplification, small)
5. Account workspace hand-rolls entity URLs instead of using `ROUTES` (simplification, small)
6. Dead `getPreferences` / `updatePreferences` client methods (cleanup, trivial)
7. Redundant per-turn work on the signed-in auto-save path (efficiency, medium)

Each sub-issue has the concrete failure scenario and a suggested fix.

Start by reading issue #1689 and then the linked sub-issues, using the account workspace as the common entry point. Review the areas named here—FavoritesProvider, legacy account routes, ROUTES, the preferences client methods, and the signed-in auto-save path. Done means all seven follow-ups are addressed without changing the scope of the original workspace PR.

Written by the indexing model from the issue text.