Sticky Settings

A cross-app concept (requested in #322): when a user makes certain choices, the choice is recorded against their profile implicitly — no settings screen, no save button. It is not shown on the profile page and is changed only by revisiting the original area of choice and choosing differently.

Mechanics

One jsonb column holds all sticky settings: users.settings (null: false, default: {}).
Each setting is exposed through store_accessor on User:
```
store_accessor :settings, :review_mode
```
The controller that owns the choice resolves it: an explicit choice in params wins and is persisted (only when it actually changed — no writes on every request); otherwise the stored value applies; otherwise the feature’s default.
Always validate the stored value against the feature’s whitelist before using it — the column is user data.

Current sticky settings

Setting	Default	Set by	Used by
`review_mode`	`read`	Tapping a mode tab (Read/Write/Listen/Speak) on the review card	`Decks::ReviewsController#show` — the Study button goes straight into review in the sticky mode; the mode-chooser page was removed
`document_voices`	`{}` (the first read-aloud plan for a language picks a random catalog voice and persists it here, #586)	The voice picker on the document reader toolbar (`PATCH /documents/:id/voice`), or implicitly by `ReadAloudPlanner`	Text-to-speech: `Resources::API::AttachAudioService` and the read-aloud/dictation planners prefer the stored voice for the language (`User#document_voice_for`); values validated against `VoiceCatalog`

Adding a new sticky setting

Add the key to the store_accessor :settings list on User.
Resolve it in the owning controller (explicit param → persist if changed; stored value → validate; fallback default).
Document it in the table above.

No migration is needed for new keys — the jsonb column absorbs them.