Rework provider selector and settings UX

Surface all enabled providers in the model selector with inline loading
and error states instead of silently hiding non-ready ones. Tap any row
to drill in; loading and error states show in the drill-down body, with
a Retry button on error. Providers with no models render a synthetic
"Default" row so every provider's drill-down has symmetric structure.

Redesign the per-provider settings sheet: search lives in the header,
discovered and custom models share one scrolling list, and the footer
hosts action buttons. Diagnostic and "Add model" each open as their own
sub-sheet, so neither expands inline or shifts the surrounding layout.

Snapshot refresh now keeps cached data visible: setQueryData replaces
the cache atomically and sibling-scope caches are invalidated (not
removed), so refreshing a provider no longer blanks its row. Search
inputs across the app are uncontrolled — onChangeText drives parent
state, and resetKey remounts the field when an explicit reset is needed
— to avoid RN's controlled-input flicker.

Adds a dev-only mock-slow provider whose discovery never resolves, so
the 30s snapshot timeout produces a real error and lets loading and
timeout-error UI be exercised end to end.

packages/app/src/stores/provider-settings-store.ts

@@ -0,0 +1,24 @@
+import { create } from "zustand";
+
+interface ProviderSettingsTarget {
+  serverId: string;
+  provider: string;
+}
+
+interface ProviderSettingsStoreState {
+  serverId: string | null;
+  provider: string | null;
+  open: (target: ProviderSettingsTarget) => void;
+  close: () => void;
+}
+
+export const useProviderSettingsStore = create<ProviderSettingsStoreState>()((set) => ({
+  serverId: null,
+  provider: null,
+  open: ({ serverId, provider }) => {
+    set({ serverId, provider });
+  },
+  close: () => {
+    set({ serverId: null, provider: null });
+  },
+}));