per-session models: every session owns its model + thinking level from birth

The reported bug (2026-09-28): every chat ran glm-5.3-flash regardless of
any pick, while composer and settings UIs agreed with each other and lied
about the run. Root cause: a fossil default-settings record (settings.json
chat section, frozen pre-slots-wave) was stamped verbatim into every new
chat at birth by the new-session flow, and resolveConversationSpec prefers
the per-chat record over the strong slot forever.

Owner redesign — models are per session, always; the strong slot is the
new-session default:

- POST /api/chats stamps the newborn verbatim from the strong slot
  (model + thinking level); no client settings PUT on create anymore
- the composer control is dual-bound: in-session it reads/writes the
  session's own record (GET /api/chats/:id, PUT .../settings); on the
  draft surfaces it binds the strong slot (picking there = changing the
  default)
- thinking levels follow the identical mechanics (confirmed by owner)
- the zombie default-settings system dies whole: useDefaultSettings,
  /api/settings/defaults, src/store/default-settings.ts, the chat
  section of UserSettings (sanitizer strips fossil copies), the legacy
  default-settings.json fold, and DEFAULT_SETTINGS_FILE
- catalogDefaultsFor collapses to the strong slot alone
- thinking display for a legacy unstamped session resolves exactly like
  the engine (session ?? strong ?? model ladder)

Tests: new chat-birth-stamp suite pins the stamp, the unconfigured boot,
the unset level, and the copy-not-live-view regression; defaults pins
rewritten strong-only; legacy-migration suite deleted.

components/stats-bar.tsx

@@ -29,8 +29,14 @@ import { ModelControl, catalogLimits, useModelCatalog } from "@/components/model
 export function StatsBar({
 	turns,
 	dictationSlot,
+	chatId,
 }: {
 	turns: TurnUsage[];
+	/** The bound session (per-session models, owner 2026-09-28): present
+	 *  on the in-session bar — the model control binds THE SESSION'S own
+	 *  record; absent on the draft surfaces, where it binds the strong
+	 *  slot (the new-session default). */
+	chatId?: string;
 	/** The dictation strip (STT wave, 2026-09-17): the row's middle is dead
 	 *  space between the session data and the model control — the mic
 	 *  button lives there, and while recording the meter unfurls to fill
@@ -169,7 +175,7 @@ export function StatsBar({
 			{/* The model + thinking control — one controller for the conversation
 			    agents (prober/teacher); the record's `turns` above still folds
 			    the ACTUAL models billed, regardless of what the picker says. */}
-			<ModelControl />
+			<ModelControl chatId={chatId} />
 		</div>
 	);
 }