Add Focus Blocker to block distracting sites

[arnelisk]

Summary

Adds a 🎯 Focus tab to the popup that blocks distracting sites with one toggle. Sites you add (X, Reddit, YouTube, TikTok, etc.) get redirected to a "get back to what matters" page while focus mode is on. Each
site has its own switch so you can keep the list and just toggle individual sites on/off without re-adding them.

Built in the same spirit as the rest of the extension — no build step, no dependencies, no network calls. State lives in chrome.storage.local (focus_enabled, focus_sites).

How it works

• background.js — webNavigation.onBeforeNavigate listener checks the host against the blocklist on top-level navigations only (frameId === 0, http(s) only) and redirects to blocked.html when focus
  mode is on.
• blocked.html — minimal static page (no JS deps) that shows which host was blocked, pulled from the query string.
• popup.{html,js} — new Focus tab with a master toggle, quick-add chips for common time sinks, a free-text input, and per-site on/off switches.
• Suffix matching — same logic as the tab cleaner's exclusion list (host === s.host || host.endsWith("." + s.host)), so adding x.com also blocks mobile.x.com.

Design choices that match the project rules

• No new permissions: reuses webNavigation (already granted for the redirect tracer) and storage.
• No new dependencies, no build step, no remote calls.
• blocked.html is declared in web_accessible_resources so chrome.runtime.getURL can navigate to it.
• Manifest bumped 2.0 → 2.1.

Test plan

• Load unpacked, open popup, switch to 🎯 Focus tab.
• Add x.com via quick-add chip; toggle focus mode on; visit https://x.com → should redirect to blocked page.
• Visit https://mobile.x.com → should also redirect (suffix match).
• Toggle the per-site switch off → x.com loads normally even with focus mode on.
• Toggle focus mode off → all sites load normally.
• Reload extension → state persists (host list + per-site toggles + master toggle).
• Adding https://www.reddit.com/r/foo normalises to reddit.com.
• Subframe / iframe navigation to a blocked host does not redirect the top-level tab.