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/querier.go

@@ -250,6 +250,7 @@ type sqlcQuerier interface {
 	GetChatIncludeDefaultSystemPrompt(ctx context.Context) (bool, error)
 	GetChatMessageByID(ctx context.Context, id int64) (ChatMessage, error)
 	GetChatMessagesByChatID(ctx context.Context, arg GetChatMessagesByChatIDParams) ([]ChatMessage, error)
+	GetChatMessagesByChatIDAscPaginated(ctx context.Context, arg GetChatMessagesByChatIDAscPaginatedParams) ([]ChatMessage, error)
 	GetChatMessagesByChatIDDescPaginated(ctx context.Context, arg GetChatMessagesByChatIDDescPaginatedParams) ([]ChatMessage, error)
 	GetChatMessagesForPromptByChatID(ctx context.Context, chatID uuid.UUID) ([]ChatMessage, error)
 	GetChatModelConfigByID(ctx context.Context, id uuid.UUID) (ChatModelConfig, error)
@@ -852,12 +853,14 @@ type sqlcQuerier interface {
 	// replicas know the worker is still alive.
 	UpdateChatHeartbeat(ctx context.Context, arg UpdateChatHeartbeatParams) (int64, error)
 	UpdateChatLabelsByID(ctx context.Context, arg UpdateChatLabelsByIDParams) (Chat, error)
+	UpdateChatLastModelConfigByID(ctx context.Context, arg UpdateChatLastModelConfigByIDParams) (Chat, error)
 	UpdateChatMCPServerIDs(ctx context.Context, arg UpdateChatMCPServerIDsParams) (Chat, error)
 	UpdateChatMessageByID(ctx context.Context, arg UpdateChatMessageByIDParams) (ChatMessage, error)
 	UpdateChatModelConfig(ctx context.Context, arg UpdateChatModelConfigParams) (ChatModelConfig, error)
 	UpdateChatPinOrder(ctx context.Context, arg UpdateChatPinOrderParams) error
 	UpdateChatProvider(ctx context.Context, arg UpdateChatProviderParams) (ChatProvider, error)
 	UpdateChatStatus(ctx context.Context, arg UpdateChatStatusParams) (Chat, error)
+	UpdateChatStatusPreserveUpdatedAt(ctx context.Context, arg UpdateChatStatusPreserveUpdatedAtParams) (Chat, error)
 	UpdateChatWorkspaceBinding(ctx context.Context, arg UpdateChatWorkspaceBindingParams) (Chat, error)
 	UpdateCryptoKeyDeletesAt(ctx context.Context, arg UpdateCryptoKeyDeletesAtParams) (CryptoKey, error)
 	UpdateCustomRole(ctx context.Context, arg UpdateCustomRoleParams) (CustomRole, error)