refactor: DRY cleanup — extract shared helpers, remove duplication across tools, LSP, overlays, and runtime

Eliminate ~500 lines of duplicate code across 31 files by extracting
shared functions, helpers, and consolidating repeated patterns.

Highlights:
- Toast helpers (toast_info/success/warning/error) on AppStateRest
- push_event() helper for turn-event queue (19 callers consolidated)
- log_write_edit_tool() shared fn (turn.rs + engine.rs ~50 lines saved)
- resolve_api_key() shared fn (spawn.rs + provider.rs)
- LSP call_positional() helper on LspClient
- lsp_cursor_params() shared schema for 4 tool files
-overlay_block() helper for consistent overlay title/border styling
- cycle_selected_index(), path_not_found/a_directory() helpers
- mark_dirty(), save_settings() on AppStateRest
- Remove redundant Err(e) => Err(e) arms in LSP tools
- Consolidate generate_workspace_tree (turn.rs → workspace.rs)
- Simplify background-review wrapper args in auto/mod.rs

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/zesdex-backend/src/app/runtime/actions/spawn.rs

@@ -57,12 +57,9 @@ pub(super) fn spawn_turn(state: &AppStateRest) {
     if messages.is_empty() {
         return;
     }
-    let mut api_key = state
-        .settings
-        .api_keys
-        .get(&state.settings.provider)
-        .cloned()
-        .unwrap_or_default();
+    let mut api_key = crate::service::provider::resolve_api_key(
+        &state.settings, &state.app_config,
+    );
     let model = state.settings.model.clone();
     let base_url = state
         .app_config
@@ -95,16 +92,6 @@ pub(super) fn spawn_turn(state: &AppStateRest) {
         }
         return;
     }
-    if api_key.is_empty() {
-        if let Some(provider_cfg) = state.app_config.providers.get(&state.settings.provider) {
-            api_key = provider_cfg
-                .api_key_env
-                .as_ref()
-                .and_then(|env| std::env::var(env).ok())
-                .or_else(|| provider_cfg.default_api_key.clone())
-                .unwrap_or_default();
-        }
-    }
     if api_key.is_empty() {
         api_key = crate::service::provider::DEFAULT_API_KEY.to_string();
     }