feat(app): polish file explorer tree with material icons and visual refinements

Replace lucide file icons with colored SVGs from material-icon-theme covering
53 language/filetype-specific icons. Add chevron expand indicators for
directories, indent guide lines, rounded rows, and restyle the header toolbar
to match the git diff pane pattern.

packages/app/package.json

@@ -111,6 +111,7 @@
     "eas-cli": "^16.24.1",
     "eslint": "^9.25.0",
     "eslint-config-expo": "~10.0.0",
+    "material-icon-theme": "^5.32.0",
     "playwright": "^1.56.1",
     "typescript": "~5.9.2",
     "vitest": "^3.2.4",