Add Dutch legal market integrations: rechtspraak.nl + wetten.overheid.nl

- New `backend/src/lib/rechtspraak.ts`: API client for Dutch case law
  (rechtspraak.nl Atom/XML) and legislation (wetten.overheid.nl SRU),
  with court code → name mapping and full-text extraction capped at 15 kB.

- `chatTools.ts`: Three new agentic tools (search_case_law, fetch_case_law,
  search_legislation) registered in RECHTSPRAAK_TOOLS; handlers in
  runToolCalls emit SSE activity events; runLLMStream accepts
  `rechtspraakEnabled` param (default ON); ParsedCitation/normalizeCitation/
  extractAnnotations extended for case_law and legislation citation types;
  system prompt extended with Dutch law search strategy and citation format.

- `chat.ts` / `projectChat.ts`: Accept `rechtspraak_enabled` from request body.

- `llm/models.ts`: Claude Sonnet 4.6 is now the default model for chat,
  tabular review, and title generation.

- `types.ts`: New AssistantEvent variants for case law / legislation activity;
  MikeCitationAnnotation extended with ecli, external_url, court, judgment_date,
  title, article optional fields; MikeMessage gains `rechtspraakEnabled`.

- `mikeApi.ts`: `rechtspraak_enabled` added to streamChat / streamProjectChat.

- `useAssistantChat.ts`: Pass rechtspraakEnabled through to API; handle four new
  SSE event types with streaming placeholders and thinking indicator.

- `ChatInput.tsx`: Scale icon toggle button (default ON) for rechtspraak.nl lookup.

- `AssistantMessage.tsx`: Render case_law_searched_start/searched, case_law_fetched,
  legislation_searched event blocks with blue dot indicators.

- `ChatView.tsx`: Citation click opens external URL in new tab when
  citation.external_url is set (case law / legislation), otherwise opens
  the document side panel as before.

- `ModelToggle.tsx`: DEFAULT_MODEL_ID changed from gemini-3-flash-preview to
  claude-sonnet-4-6.

https://claude.ai/code/session_016JxWnFc3baeDgkzDgxY6ex