Add external editor/file picker, update theme

Introduce file/application pickers and external-editor integration, plus a visual refresh. Frontend: add UI for selecting/rescanning installed IDEs, custom editor input, "Open in File Explorer" and "Open in Code Editor" actions, clone destination browse, helper utilities, new icons, and many CSS theme/UX improvements (variables, shadows, scrollbars, selection, refined component styles). State: track installedIdes and scan status. Tauri API: expose browseDirectory, browseApplication and scanInstalledIdes, and wire UI handlers to call them. Backend: add InstalledIde struct and update tauri Cargo manifest and capabilities to allow dialogs. Overall improves editor/workflow integrations and modernizes the app styling.

frontend/js/state.js

@@ -16,6 +16,9 @@ const state = {
   localRepoScanResults: [],
   localRepoScanLoading: false,
   localRepoScanError: "",
+  installedIdes: [],
+  installedIdeScanLoading: false,
+  installedIdeScanError: "",
   cloneUrlInput: "",
   cloneDestinationInput: "",
   commitMessage: "",