refactor: remove Anthropic API integration, CLI agent only (ADR-0028)

Remove AIChatPanel, useAIChat hook, Rust ai_chat command, and
anthropic_key from settings. AI is now exclusively via Claude CLI
subprocess (AiPanel). Simplifies codebase and eliminates API key
management for users. ADR-0027 superseded.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/menu.rs

@@ -293,7 +293,7 @@ fn build_note_menu(app: &App) -> MenuResult {
         .id(EDIT_TOGGLE_RAW_EDITOR)
         .accelerator("CmdOrCtrl+\\")
         .build(app)?;
-    let toggle_ai_chat = MenuItemBuilder::new("Toggle AI Chat")
+    let toggle_ai_chat = MenuItemBuilder::new("Toggle AI Panel")
         .id(VIEW_TOGGLE_AI_CHAT)
         .accelerator("CmdOrCtrl+I")
         .build(app)?;