refactor: Add SPA feed detection and improve YouTube channel feed by @mt190502 in issue 22

fix(parser): Remove YouTube feed detection and simplify passive feed by @mt190502 in issue 21

Breaking changes
- Extended Scan Exclusions — Common sites moved out of Ignored URLs. They no longer block all detection; active origin probing is skipped while passive detection and Feed Templates still run.
- Exact Host matching — subdomain mode now matches only the exact hostname, not subdomains of it.
New
- Feed Templates — Generate feed URLs from page URLs with named placeholders. Work even when Extended Feed Scan is off.
- Browser Sync — Sync your settings through Google Sync or Mozilla Sync. Enable it separately on each installation.
- Import / Export — Download or restore settings as a JSON file.
- Migration backup — Your pre-upgrade settings are backed up automatically. Download from the migration notice or Import / Export section; restore anytime with Import.
Improvements
- Extended Feed Scan runs in the background and caches results per origin (24 h positive, 15 min negative).
- Passive detection makes no network requests.
- Theme list loads on demand when you open the selector.
- Accessibility: better keyboard navigation, live status updates, and ARIA labels.

Update dependencies

- Fixed dark/light theme foreground/background compatibility

- Removed file extension in settings

- Fix overflow issue for long feed urls

- Added custom theme support

- Fixed default variable setting in settings when an option is undefined

- Improved extended feed scan, added this setting in settings page

Extension Functionality Enhancements:
- Implemented new background script functionalities including feed detection, icon state management, and settings initialization.
- Applied theme settings and added a settings button to the popup.
- Added theme and ignored sites management in the settings page.

Code Quality Improvements:
- Switched to webpack infrastructure