Add code actions (Phase 2): quick fixes and refactorings via LSP

- CodeActionsState machine: Idle → Requesting → Available (with menu)
- 200ms debounced fetch on cursor/selection change
- Cmd+. opens the code actions menu
- Menu renders as positioned overlay with action titles and kind labels
- apply_workspace_edit helper applies LSP WorkspaceEdits to the editor
- Wired into SelectionEnd event for automatic background fetching

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

app/src/code/mod.rs

@@ -6,6 +6,8 @@ use std::any::Any;
 use std::fmt::Debug;
 use std::ops::AddAssign;
 
+#[cfg(not(target_family = "wasm"))]
+pub mod code_actions;
 #[cfg(not(target_family = "wasm"))]
 pub mod completion;
 #[cfg(not(target_family = "wasm"))]