Add media browser feature with UI improvements

- Refactored index.html: Split into separate HTML (309 lines), CSS (908 lines), and JS (1,286 lines) files
- Implemented media browser with folder configuration, recursive navigation, and thumbnail display
- Added metadata extraction using mutagen library (title, artist, album, duration, bitrate, codec)
- Implemented thumbnail generation and caching with SHA256 hash-based keys and LRU eviction
- Added platform-specific file playback (os.startfile on Windows, xdg-open on Linux, open on macOS)
- Implemented path validation security to prevent directory traversal attacks
- Added smooth thumbnail loading with fade-in animation and loading spinner
- Added i18n support for browser (English and Russian)
- Updated dependencies: mutagen>=1.47.0, pillow>=10.0.0
- Added comprehensive media browser documentation to README

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

media_server/static/locales/en.json

@@ -107,5 +107,29 @@
   "callbacks.msg.load_failed": "Failed to load callback details",
   "callbacks.msg.list_failed": "Failed to load callbacks",
   "callbacks.confirm.delete": "Are you sure you want to delete the callback \"{name}\"?",
-  "callbacks.confirm.unsaved": "You have unsaved changes. Are you sure you want to discard them?"
+  "callbacks.confirm.unsaved": "You have unsaved changes. Are you sure you want to discard them?",
+  "browser.title": "Media Browser",
+  "browser.manage_folders": "Manage Folders",
+  "browser.select_folder": "Select a folder...",
+  "browser.select_folder_option": "Select a folder...",
+  "browser.no_folder_selected": "Select a folder to browse media files",
+  "browser.no_items": "No media files found in this folder",
+  "browser.previous": "Previous",
+  "browser.next": "Next",
+  "browser.play_success": "Playing {filename}",
+  "browser.play_error": "Failed to play file",
+  "browser.error_loading": "Error loading directory",
+  "browser.error_loading_folders": "Failed to load media folders",
+  "browser.manage_folders_hint": "Folder management coming soon! For now, edit config.yaml to add media folders.",
+  "browser.folder_dialog.title_add": "Add Media Folder",
+  "browser.folder_dialog.title_edit": "Edit Media Folder",
+  "browser.folder_dialog.folder_id": "Folder ID *",
+  "browser.folder_dialog.folder_id_help": "Alphanumeric and underscore only",
+  "browser.folder_dialog.label": "Label *",
+  "browser.folder_dialog.label_help": "Display name for this folder",
+  "browser.folder_dialog.path": "Path *",
+  "browser.folder_dialog.path_help": "Absolute path to media directory",
+  "browser.folder_dialog.enabled": "Enabled",
+  "browser.folder_dialog.cancel": "Cancel",
+  "browser.folder_dialog.save": "Save"
 }