feat(ai): add Anthropic Claude adapter as primary AI provider

Wire up Claude Messages API with SSE streaming, supporting the
different event format (content_block_delta) vs OpenRouter's
OpenAI-compatible format. Claude is the default when its API key
is present.

- Dual provider system: Anthropic (direct) and OpenRouter
- Claude streams via content_block_delta events, OpenRouter via
  choices[0].delta.content
- Model picker dropdown in chat header (click model name to switch)
- Available models: Claude Sonnet 4, Opus 4, Haiku 3.5,
  Llama 4 Maverick/Scout (free), Mistral Small 3.1 (free)
- Sidebar status shows active provider name
- anthropic-dangerous-direct-browser-access header for browser use

Made-with: Cursor

.env.example

@@ -5,6 +5,10 @@
 # Get your key at: https://openrouter.ai/keys
 VITE_OPENROUTER_API_KEY=sk-or-your-key-here
 
+# Anthropic Claude (direct API access)
+# Get your key at: https://console.anthropic.com/settings/keys
+VITE_ANTHROPIC_API_KEY=sk-ant-your-key-here
+
 # TMDB API (free, for real film poster images)
 # Get your key at: https://www.themoviedb.org/settings/api
 VITE_TMDB_API_KEY=your-tmdb-key-here