Refactor: ES module architecture, tauri-plugin-store, unified design

Major changes:
- ES module system: main.js as central hub, shared modules
- tauri-plugin-store for persistent storage (dialog, fs plugins added)
- tauri-plugin-dialog for folder selection
- tauri-plugin-fs for real file system scanning
- Removed MOCK_FILES/FILE_TREE - real file scanner implemented
- All pages use unified page-header + page-content layout
- Custom titlebar with window controls
- Onboarding: 4-step flow with folder selection (Downloads default)
- CSS: unified design system, .hidden utility class added
- SVG icons throughout (no emoji)

Note: onboarding.js downloadDir() has runtime issue - needs fix

.gitignore

@@ -4,11 +4,6 @@ node_modules/
 # Build output
 dist/
 
-# Tauri
-src-tauri/target/
-src-tauri/gen/schemas
-src-tauri/Cargo.lock
-
 # Environment files
 .env
 .env.*
@@ -34,3 +29,20 @@ Thumbs.db
 
 # Sisyphus session files
 .sisyphus/
+
+# Vite
+.vite/
+
+# TailwindCSS / PostCSS
+tailwind.config.js
+postcss.config.js
+*.config.js
+
+# Tauri
+src-tauri/target/
+src-tauri/gen/schemas
+src-tauri/Cargo.lock
+src-tauri/.tauriignore
+
+# Debug
+*.tsbuildinfo