feat: provider model freshness TTL and diagnostic UI (#426)

* feat: add server-side TTL for provider snapshots and model list UI

Provider snapshots cached forever, causing newly available models
(e.g. OpenCode Go minimax, glm) to never appear in the picker.

Server: ProviderSnapshotManager now returns stale data immediately
and kicks off a background re-warm when the snapshot is older than
5 minutes. Injectable TTL/clock for testing.

App: Provider settings rows show model count and are tappable to
open a diagnostic sheet with a read-only model list (label + ID).

* feat: add per-provider refresh for models and diagnostic

* refactor: await refresh completion and clean up diagnostic sheet state

- ProviderSnapshotManager.refresh is now async; session.ts awaits it so
  the RPC ACK reflects real completion instead of just queuing work.
- Preserve models/modes/fetchedAt on entries during targeted refresh so
  the list no longer flashes empty mid-refresh.
- Show "Updated Xs ago" next to the Models count, plus loading and
  error states for the list body.
- Split resetSnapshotToLoading into full vs targeted branches.
- Flatten nested ternary in model list rendering into renderModelsBody.
- Drop redundant local refreshing state and cargo useMemo wrappers.

* fix(app): widen isImeComposingKeyboardEvent type to accept optional fields

TextInputKeyPressEventData has isComposing and keyCode as optional, so
Pick<KeyboardEvent, ...> was too narrow and broke typecheck on main.

packages/server/src/shared/messages.ts

@@ -852,6 +852,7 @@ export const GetProvidersSnapshotRequestMessageSchema = z.object({
 export const RefreshProvidersSnapshotRequestMessageSchema = z.object({
   type: z.literal("refresh_providers_snapshot_request"),
   cwd: z.string().optional(),
+  providers: z.array(AgentProviderSchema).optional(),
   requestId: z.string(),
 });