fix: add missing Toaster component to enable toast display (#65)

The project was calling toast.success/error/warning from sonner in 7 files,
but the <Toaster> component was never rendered in the React tree. This meant
sonner stored toast data internally but no UI was ever displayed to users.

Changes:
- Create src/components/ui/sonner.tsx with shadcn/ui-style Toaster wrapper
  that integrates with ThemeModeProvider for light/dark theme support
- Add <Toaster /> to AppRoot.tsx so it works across all entry points
  (popup, sidepanel, browser-tab)

fix: always convert dots to underscores in i18n t() key lookup

The t() function only converted dots to underscores when the key contained
a colon (namespace:key format). Keys using dot notation like
'messages.copySuccess' were passed directly to chrome.i18n.getMessage()
without dot-to-underscore conversion, causing lookups to fail silently and
return empty strings - resulting in toast notifications with no text.

Now all separators (both ':' and '.') are consistently converted to '_'
regardless of format, matching the messages.json key convention.

fix: position toast at bottom-center instead of bottom-right

src/utils/chromeI18n.ts

@@ -26,12 +26,8 @@ export function useI18n(namespace?: string | string[]) {
   const namespaces = Array.isArray(namespace) ? namespace : namespace ? [namespace] : [];
 
   const t = (key: string, options?: Record<string, unknown>): string => {
-    let msgId = key;
-
-    // 处理 namespace:key 格式（兼容原 i18next 用法）
-    if (key.includes(':')) {
-      msgId = key.replace(':', '_').replace(/\./g, '_');
-    }
+    // 统一将分隔符转换为下划线，兼容 'namespace:key.path' 和 'key.path' 两种写法
+    const msgId = key.replace(':', '_').replace(/\./g, '_');
 
     // 先尝试直接查找 key
     let message = getMessage(msgId);