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/dbmetrics/querymetrics.go

@@ -1144,6 +1144,14 @@ func (m queryMetricsStore) GetChatMessagesByChatID(ctx context.Context, chatID d
 	return r0, r1
 }
 
+func (m queryMetricsStore) GetChatMessagesByChatIDAscPaginated(ctx context.Context, arg database.GetChatMessagesByChatIDAscPaginatedParams) ([]database.ChatMessage, error) {
+	start := time.Now()
+	r0, r1 := m.s.GetChatMessagesByChatIDAscPaginated(ctx, arg)
+	m.queryLatencies.WithLabelValues("GetChatMessagesByChatIDAscPaginated").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "GetChatMessagesByChatIDAscPaginated").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) GetChatMessagesByChatIDDescPaginated(ctx context.Context, arg database.GetChatMessagesByChatIDDescPaginatedParams) ([]database.ChatMessage, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetChatMessagesByChatIDDescPaginated(ctx, arg)
@@ -4096,6 +4104,14 @@ func (m queryMetricsStore) UpdateChatLabelsByID(ctx context.Context, arg databas
 	return r0, r1
 }
 
+func (m queryMetricsStore) UpdateChatLastModelConfigByID(ctx context.Context, arg database.UpdateChatLastModelConfigByIDParams) (database.Chat, error) {
+	start := time.Now()
+	r0, r1 := m.s.UpdateChatLastModelConfigByID(ctx, arg)
+	m.queryLatencies.WithLabelValues("UpdateChatLastModelConfigByID").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "UpdateChatLastModelConfigByID").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) UpdateChatMCPServerIDs(ctx context.Context, arg database.UpdateChatMCPServerIDsParams) (database.Chat, error) {
 	start := time.Now()
 	r0, r1 := m.s.UpdateChatMCPServerIDs(ctx, arg)
@@ -4144,6 +4160,14 @@ func (m queryMetricsStore) UpdateChatStatus(ctx context.Context, arg database.Up
 	return r0, r1
 }
 
+func (m queryMetricsStore) UpdateChatStatusPreserveUpdatedAt(ctx context.Context, arg database.UpdateChatStatusPreserveUpdatedAtParams) (database.Chat, error) {
+	start := time.Now()
+	r0, r1 := m.s.UpdateChatStatusPreserveUpdatedAt(ctx, arg)
+	m.queryLatencies.WithLabelValues("UpdateChatStatusPreserveUpdatedAt").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "UpdateChatStatusPreserveUpdatedAt").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) UpdateChatWorkspaceBinding(ctx context.Context, arg database.UpdateChatWorkspaceBindingParams) (database.Chat, error) {
 	start := time.Now()
 	r0, r1 := m.s.UpdateChatWorkspaceBinding(ctx, arg)