Mainly i18n updates and refactoring

Dark Mode (#16)

Full dark mode support with automatic system theme detection.

Added

• Close duplicate tabs - Optional preference to close duplicate tabs when sorting (keeps the leftmost tab). Matches normalized URLs (ignores hash, query string, www, case) and the same issue-tracker resource when the URL shape differs. Closes #17. By @manosnoam
• Ignore www prefix when sorting tabs by URL - Closes #20. By @AntonDobrovinskiy
• Auto theme (follows system preference) is the default for new users
• Automatic system theme detection (follows OS preference)
• Manual theme selection: Auto, Dark, or Light
• New "Appearance" section in preferences
• Full localization support for English and French

Changed

• CSS Architecture: Refactored to use CSS custom properties for theming

Technical Improvements

• CSS Variables: Theme colors defined using CSS custom properties (:root and [data-theme="dark"])
• System Detection: Uses prefers-color-scheme media query for auto theme detection
• Instant Updates: Theme changes are applied immediately without page reload

Files Changed

• template-extension/tab-sorter.js - Theme storage and retrieval
• template-extension/popup-tab-sorter.js - Theme UI and application logic
• template-extension/tab-sorter.css - Dark mode styles with CSS variables
• template-extension/_locales/en/messages.json - English translations
• template-extension/_locales/fr/messages.json - French translations
• Added test-dark-mode-issue-16.md - Comprehensive testing documentation

Sort Pinned Tabs (#9)

New option to sort pinned tabs independently.

Added

• New "Also sort pinned tabs" checkbox in General Preferences (disabled by default)
• Pinned tabs are sorted among themselves while remaining in the pinned area
• Works with all sorting methods (URL, title, MRU, favicon)
• Compatible with suspended tabs grouping feature
• Full localization support for English and French

Technical Improvements

• Pinned Tabs Detection: Separates pinned and non-pinned tabs before sorting
• Independent Sorting: New sortPinnedTabsOnly() function for sorting pinned tabs separately

Files Changed

• template-extension/tab-sorter.js - Pinned tabs sorting logic
• template-extension/popup-tab-sorter.js - UI updates for new checkbox
• template-extension/_locales/en/messages.json - English translations
• template-extension/_locales/fr/messages.json - French translations
• Added test-pinned-tabs-issue-9.md - Comprehensive testing documentation

Suspended Tabs Position Control ([#18](