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.

src/core/settings-resolution.ts

@@ -68,9 +68,6 @@ export type EffectiveAppearance = FactorySettings["appearance"];
 /** The concrete view: every knob resolved, no absent fields. */
 export interface EffectiveSettings {
 	general: Required<GeneralSettings>;
-	/** The default ChatSettings for new sessions — passthrough delta
-	 *  ({} = no default = BYOK "no model" first boot). */
-	chat: ChatSettings;
 	agent: {
 		models: Record<BackgroundModelRole, EffectiveModelSpec | undefined>;
 		/** Delta map as stored — consumers resolve absent key → enabled. */
@@ -175,7 +172,6 @@ export function resolveSettings(record: UserSettings | null): EffectiveSettings
 		general: {
 			autoUpdate: record?.general?.autoUpdate ?? FACTORY_SETTINGS.general.autoUpdate,
 		},
-		chat: record?.chat ?? {},
 		agent: {
 			models,
 			tools: agent.tools ?? {},