Add signature help (Phase 4): parameter hints on function calls

- SignatureHelpState: None → Loading → Showing
- Triggers on '(' and ',' characters, dismisses on ')'
- Renders tooltip above cursor with active parameter highlighted
- Active parameter shown in warning color (bold yellow/orange)
- Includes documentation display when available from LSP
- Dismissed on Escape along with other LSP overlays

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

app/src/code/mod.rs

@@ -13,6 +13,8 @@ pub mod completion;
 #[cfg(not(target_family = "wasm"))]
 pub mod find_references_view;
 #[cfg(not(target_family = "wasm"))]
+pub mod signature_help;
+#[cfg(not(target_family = "wasm"))]
 pub mod language_server_extension;
 #[cfg_attr(not(target_family = "wasm"), path = "local_code_editor.rs")]
 #[cfg_attr(target_family = "wasm", path = "local_code_editor_wasm.rs")]