feat: draft settings persistence — shared model/thinking/context record for the new-session surfaces

Both new-session pages (the home graph view's topic dock and /chat/new)
held their model + thinking + knowledge-context choices in plain
useState(undefined) — every navigation reset them to config defaults.

Now both surfaces read/write ONE server-side record
(data/draft-settings.json via GET/PUT /api/settings/draft, the
render-settings pattern): whatever the learner last left the controls on
is what every later visit shows. useDraftSettings loads on mount with a
touch-guard (a slow GET never clobbers a fast choice), saves
optimistically. Session creation still carries the choice into the new
chat exactly as before; per-session settings are untouched.

The PUT validation is the exact per-chat contract — parseSettingsBody
extracted verbatim from app/api/chats/[id]/settings and shared by both
routes (unknown model/level → 400 with the supported list, never a
silent clamp). 400s on garbage, {} = the clear-overrides write.

Live-verified: Cmd+K+Enter model pick, Shift+Tab thinking cycle, context
slider — all held across home → session → home → /chat/new, and carried
into a created session's record.

src/server/index.ts

@@ -11,6 +11,8 @@ export * from "./commit-positions.ts";
 export * from "./jobs.ts";
 export * from "./stores.ts";
 export { readRenderSettings, writeRenderSettings } from "../store/render-settings.ts";
+export { readDraftSettings, writeDraftSettings } from "../store/draft-settings.ts";
+export * from "./settings-body.ts";
 export * from "./turn-liveness.ts";
 export * from "./observability.ts";
 export * from "./model-catalog.ts";