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}