Comprehensive WebUI improvements: security, UX, accessibility, performance

Security:
- Replace inline onclick handlers with data-attribute event delegation (XSS fix)
- Remove auth tokens from URL query params; use Authorization header + blob URLs
- Defer artwork blob URL revocation to prevent ERR_FILE_NOT_FOUND

Reliability:
- Merge duplicate DOMContentLoaded listeners
- WebSocket exponential backoff reconnect (3s base, 30s max, 20 attempts)
- Connection banner with manual reconnect button after failures

UX:
- Toast notifications now stack (multiple visible simultaneously)
- Custom styled confirm dialog replacing native confirm()
- Drag-to-seek on progress bars (mouse + touch)
- Keyboard shortcuts: Space, arrows, M for media controls
- Browser search matches both filename and title
- Path separator auto-detection (Unix/Windows)

Accessibility:
- WAI-ARIA Tabs pattern (tablist, tab, tabpanel roles)
- Arrow/Home/End keyboard navigation in tab bar
- ARIA slider roles on progress bars with live value updates
- aria-label on volume sliders, aria-live on status dot

Performance:
- Thumbnail cache (Map, max 200 entries, LRU eviction)
- Skip revocation of cached blob URLs during grid re-render
- Blob URL cleanup on page unload

Visual polish:
- Vinyl mode uses CSS custom properties (works in light + dark themes)
- Light theme shadow overrides for containers, dialogs, toasts
- Optimized system font stack

Code quality:
- Scoped button reset, merged duplicate CSS selectors
- WCAG AA contrast fix for --text-muted
- Normalized CSS to consistent 4-space indentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

media_server/static/locales/en.json

@@ -154,6 +154,12 @@
   "browser.folder_dialog.enabled": "Enabled",
   "browser.folder_dialog.cancel": "Cancel",
   "browser.folder_dialog.save": "Save",
+  "browser.download_error": "Failed to download file",
+  "connection.reconnecting": "Connection lost. Reconnecting (attempt {attempt})...",
+  "connection.lost": "Connection lost. Server may be unavailable.",
+  "connection.reconnect": "Reconnect",
+  "dialog.cancel": "Cancel",
+  "dialog.confirm": "Confirm",
   "footer.created_by": "Created by",
   "footer.source_code": "Source Code"
 }

media_server/static/locales/ru.json

@@ -154,6 +154,12 @@
   "browser.folder_dialog.enabled": "Включено",
   "browser.folder_dialog.cancel": "Отмена",
   "browser.folder_dialog.save": "Сохранить",
+  "browser.download_error": "Не удалось скачать файл",
+  "connection.reconnecting": "Соединение потеряно. Переподключение (попытка {attempt})...",
+  "connection.lost": "Соединение потеряно. Сервер может быть недоступен.",
+  "connection.reconnect": "Переподключиться",
+  "dialog.cancel": "Отмена",
+  "dialog.confirm": "Подтвердить",
   "footer.created_by": "Создано",
   "footer.source_code": "Исходный код"
 }