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/server/src/server/agent/provider-manifest.ts

@@ -151,6 +151,16 @@ const MOCK_LOAD_TEST_MODES: AgentProviderModeDefinition[] = [
   },
 ];
 
+const MOCK_SLOW_MODES: AgentProviderModeDefinition[] = [
+  {
+    id: "default",
+    label: "Default",
+    description: "Dev-only mode for the mock slow provider",
+    icon: "ShieldOff",
+    colorTier: "dangerous",
+  },
+];
+
 export const AGENT_PROVIDER_DEFINITIONS: AgentProviderDefinition[] = [
   {
     id: "claude",
@@ -212,6 +222,13 @@ export const DEV_AGENT_PROVIDER_DEFINITIONS: AgentProviderDefinition[] = [
     defaultModeId: "load-test",
     modes: MOCK_LOAD_TEST_MODES,
   },
+  {
+    id: "mock-slow",
+    label: "Mock Slow Provider",
+    description: "Dev-only: hangs during model discovery to test loading and timeout UI",
+    defaultModeId: "default",
+    modes: MOCK_SLOW_MODES,
+  },
 ];
 
 export function getAgentProviderDefinition(

packages/server/src/server/agent/provider-registry.ts

@@ -31,6 +31,7 @@ import { OpenCodeAgentClient } from "./providers/opencode-agent.js";
 import { OpenCodeServerManager } from "./providers/opencode/server-manager.js";
 import { PiRpcAgentClient } from "./providers/pi/agent.js";
 import { MockLoadTestAgentClient } from "./providers/mock-load-test-agent.js";
+import { MockSlowProviderClient } from "./providers/mock-slow-provider.js";
 import {
   AGENT_PROVIDER_DEFINITIONS,
   BUILTIN_PROVIDER_IDS,
@@ -126,6 +127,7 @@ const PROVIDER_CLIENT_FACTORIES: Record<string, ProviderClientFactory> = {
       runtimeSettings,
     }),
   mock: (logger) => new MockLoadTestAgentClient(logger),
+  "mock-slow": () => new MockSlowProviderClient(),
 };
 
 function getCursorACPCommand(

packages/server/src/server/agent/providers/mock-slow-provider.ts

@@ -0,0 +1,78 @@
+import type {
+  AgentCapabilityFlags,
+  AgentClient,
+  AgentLaunchContext,
+  AgentMode,
+  AgentModelDefinition,
+  AgentPersistenceHandle,
+  AgentProvider,
+  AgentSession,
+  AgentSessionConfig,
+  ListModelsOptions,
+  ListModesOptions,
+  ListPersistedAgentsOptions,
+  PersistedAgentDescriptor,
+} from "../agent-sdk-types.js";
+
+export const MOCK_SLOW_PROVIDER_ID = "mock-slow";
+
+const CAPABILITIES: AgentCapabilityFlags = {
+  supportsStreaming: false,
+  supportsSessionPersistence: false,
+  supportsDynamicModes: false,
+  supportsMcpServers: false,
+  supportsReasoningStream: false,
+  supportsToolInvocations: false,
+  supportsRewindConversation: false,
+  supportsRewindFiles: false,
+  supportsRewindBoth: false,
+};
+
+function neverResolves<T>(): Promise<T> {
+  return new Promise<T>(() => {});
+}
+
+export class MockSlowProviderClient implements AgentClient {
+  readonly provider: AgentProvider = MOCK_SLOW_PROVIDER_ID;
+  readonly capabilities = CAPABILITIES;
+
+  async isAvailable(): Promise<boolean> {
+    return true;
+  }
+
+  listModels(_options: ListModelsOptions): Promise<AgentModelDefinition[]> {
+    return neverResolves<AgentModelDefinition[]>();
+  }
+
+  listModes(_options: ListModesOptions): Promise<AgentMode[]> {
+    return neverResolves<AgentMode[]>();
+  }
+
+  async listPersistedAgents(
+    _options?: ListPersistedAgentsOptions,
+  ): Promise<PersistedAgentDescriptor[]> {
+    return [];
+  }
+
+  async getDiagnostic(): Promise<{ diagnostic: string }> {
+    return {
+      diagnostic:
+        "Mock slow provider: dev-only. listModels() never resolves so the snapshot manager will time out.",
+    };
+  }
+
+  createSession(
+    _config: AgentSessionConfig,
+    _launchContext?: AgentLaunchContext,
+  ): Promise<AgentSession> {
+    throw new Error("Mock slow provider is dev-only; sessions are not supported.");
+  }
+
+  resumeSession(
+    _handle: AgentPersistenceHandle,
+    _overrides?: Partial<AgentSessionConfig>,
+    _launchContext?: AgentLaunchContext,
+  ): Promise<AgentSession> {
+    throw new Error("Mock slow provider is dev-only; sessions are not supported.");
+  }
+}