Fix Chrome MV3 blocking, add proxy detection, switch to per-site permissions

[adriaandotcom]

Summary

Closes #5

• Fixes Chrome breakage. IS_FIREFOX detection no longer trips on Chrome's new browser global; Chrome stays on the declarativeNetRequest path.
• Proxy detection. On Block, detects and blocks */simple.gif beacons (via performance.getEntriesByType) and same-origin .js proxy scripts (via /* Simple Analytics body signature). Added "image" to DNR resource types.
• Removes the counter. Badge count and per-script timesBlocked never worked reliably under DNR; all related code and UI copy is gone.
• Permission model: "selected websites". host_permissions: ["<all_urls>"] → optional_host_permissions: ["*://*/*"]. No more "on all sites" warning; users grant access per site.
• Popup-dies-on-permission-dialog fix. The popup now stores a pendingBlock intent and a new chrome.permissions.onAdded listener in the background finishes detection + storage + icon refresh. First-click Block now works.
• Migration UX. On upgrade, if saved blocked sites have no host access, the options page opens automatically and shows a banner with a Re-enable button per site.
• Options page polish. Max width, card + shadow, SA logo, dark mode, updated copy.
• Popup polish. Link to the options page.
• DNR reliability. Real error messages (no more [object Object]), dedup + empty-filter guards, storage.onChanged is the single source of truth (killed a rule-id race), non-blocklist keys no longer crash the listener.
• Noise removal. Silenced No tab with id errors.
• Docs. README rewritten with build + release steps (Simple Analytics Google account).
• Version bump. 1.8 → 1.9.

Security implications

• No security impact
• Has security impact - described as:

Testing

Locally in Google Chrome.

Test plan

• Standard SA site: Block → allow → reload → icon gray, SA script blocked.
• Proxy site: Block → allow → reload → */simple.gif + detected proxy .js blocked.
• Upgrade from 1.8 with existing sites: options page opens, Re-enable per site restores blocking.
• Popup "the options page" link opens options and closes popup.

Checklist

• Linked to an issue
• Tested
• Asked for a review