feat: implement full breadcrumb path navigation in Files tab

- Add breadcrumbPath array to app state for tracking folder hierarchy
- Rewrite updateBreadcrumb() to render full path: Home > folder > subfolder
- Each breadcrumb segment is clickable to navigate back to that level
- Current folder shown in bold (non-clickable), parent folders as links
- Reset breadcrumb path on tab switch, home navigation, and initial load
- Update navigateFolder/selectFolder to push to breadcrumb path
- Enhanced breadcrumb CSS with hover effects and dark mode support

static/css/base/variables.css

@@ -0,0 +1,9 @@
+:root {
+    --color-bg-page: #f5f7fa;
+    --color-bg-surface: #ffffff;
+    --color-border: #e2e8f0;
+    --color-text: #2d3748;
+    --color-text-muted: #718096;
+    --color-accent: #ff5e3a;
+    --color-accent-hover: #e04520;
+}