feat: add manual chat title regeneration (#23633)

## Summary

Adds a "Generate new title" action that lets users manually regenerate a
chat's title using richer conversation context than the automatic
first-message title path.

## Changes

### Backend
- **New endpoint:** `POST
/api/experimental/chats/{chatID}/title/regenerate` returns the updated
Chat with a regenerated title
- **Manual title algorithm:** Extracts useful user/assistant text turns
→ selects first user turn + last 3 turns → builds context with gap
markers → renders prompt with anti-recency guidance → calls lightweight
model → normalizes output
- **Helpers:** `extractManualTitleTurns`,
`selectManualTitleTurnIndexes`, `buildManualTitleContext`,
`renderManualTitlePrompt`, `generateManualTitle` — all private, with the
public `Server.RegenerateChatTitle` method
- **SDK:** `ExperimentalClient.RegenerateChatTitle(ctx, chatID) (Chat,
error)`
- Persists title via existing `UpdateChatByID` and broadcasts
`ChatEventKindTitleChange`

### Frontend
- API client method + React Query mutation with cache invalidation
- "Generate new title" menu item (with wand icon) in both TopBar and
Sidebar dropdown menus
- Loading/disabled state while regeneration is in-flight
- Error toast on failure
- Stories updated for both menus

### Tests
- `quickgen_test.go`: Table-driven tests for all 4 helper functions
(turn extraction, index selection, context building, prompt rendering)
- `exp_chats_test.go`: Handler tests (ChatNotFound,
NotFoundForDifferentUser, NoDaemon)

## Design notes
- The existing auto-title path (`maybeGenerateChatTitle`, `titleInput`)
is completely unchanged
- Manual regeneration uses richer context (first user turn + last 3
turns + gap markers) vs the auto path's single first message
- Endpoint is experimental and marked with `@x-apidocgen {"skip": true}`

coderd/database/dbmock/dbmock.go

@@ -2103,6 +2103,21 @@ func (mr *MockStoreMockRecorder) GetChatMessagesByChatID(ctx, arg any) *gomock.C
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatMessagesByChatID", reflect.TypeOf((*MockStore)(nil).GetChatMessagesByChatID), ctx, arg)
 }
 
+// GetChatMessagesByChatIDAscPaginated mocks base method.
+func (m *MockStore) GetChatMessagesByChatIDAscPaginated(ctx context.Context, arg database.GetChatMessagesByChatIDAscPaginatedParams) ([]database.ChatMessage, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetChatMessagesByChatIDAscPaginated", ctx, arg)
+	ret0, _ := ret[0].([]database.ChatMessage)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetChatMessagesByChatIDAscPaginated indicates an expected call of GetChatMessagesByChatIDAscPaginated.
+func (mr *MockStoreMockRecorder) GetChatMessagesByChatIDAscPaginated(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatMessagesByChatIDAscPaginated", reflect.TypeOf((*MockStore)(nil).GetChatMessagesByChatIDAscPaginated), ctx, arg)
+}
+
 // GetChatMessagesByChatIDDescPaginated mocks base method.
 func (m *MockStore) GetChatMessagesByChatIDDescPaginated(ctx context.Context, arg database.GetChatMessagesByChatIDDescPaginatedParams) ([]database.ChatMessage, error) {
 	m.ctrl.T.Helper()
@@ -7745,6 +7760,21 @@ func (mr *MockStoreMockRecorder) UpdateChatLabelsByID(ctx, arg any) *gomock.Call
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatLabelsByID", reflect.TypeOf((*MockStore)(nil).UpdateChatLabelsByID), ctx, arg)
 }
 
+// UpdateChatLastModelConfigByID mocks base method.
+func (m *MockStore) UpdateChatLastModelConfigByID(ctx context.Context, arg database.UpdateChatLastModelConfigByIDParams) (database.Chat, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "UpdateChatLastModelConfigByID", ctx, arg)
+	ret0, _ := ret[0].(database.Chat)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// UpdateChatLastModelConfigByID indicates an expected call of UpdateChatLastModelConfigByID.
+func (mr *MockStoreMockRecorder) UpdateChatLastModelConfigByID(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatLastModelConfigByID", reflect.TypeOf((*MockStore)(nil).UpdateChatLastModelConfigByID), ctx, arg)
+}
+
 // UpdateChatMCPServerIDs mocks base method.
 func (m *MockStore) UpdateChatMCPServerIDs(ctx context.Context, arg database.UpdateChatMCPServerIDsParams) (database.Chat, error) {
 	m.ctrl.T.Helper()
@@ -7834,6 +7864,21 @@ func (mr *MockStoreMockRecorder) UpdateChatStatus(ctx, arg any) *gomock.Call {
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatStatus", reflect.TypeOf((*MockStore)(nil).UpdateChatStatus), ctx, arg)
 }
 
+// UpdateChatStatusPreserveUpdatedAt mocks base method.
+func (m *MockStore) UpdateChatStatusPreserveUpdatedAt(ctx context.Context, arg database.UpdateChatStatusPreserveUpdatedAtParams) (database.Chat, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "UpdateChatStatusPreserveUpdatedAt", ctx, arg)
+	ret0, _ := ret[0].(database.Chat)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// UpdateChatStatusPreserveUpdatedAt indicates an expected call of UpdateChatStatusPreserveUpdatedAt.
+func (mr *MockStoreMockRecorder) UpdateChatStatusPreserveUpdatedAt(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatStatusPreserveUpdatedAt", reflect.TypeOf((*MockStore)(nil).UpdateChatStatusPreserveUpdatedAt), ctx, arg)
+}
+
 // UpdateChatWorkspaceBinding mocks base method.
 func (m *MockStore) UpdateChatWorkspaceBinding(ctx context.Context, arg database.UpdateChatWorkspaceBindingParams) (database.Chat, error) {
 	m.ctrl.T.Helper()