Add browser domain overrides

[FuJacob]

Summary

Adds browser domain overrides on top of the existing app-level enable/disable rules so Tabby can behave differently per site inside Chrome-, Safari-, and other browser-based editors. The change threads focused-tab URL detection through the focus snapshot pipeline, persists explicit domain rules with registrable-domain default matching plus exact-host opt-in, and exposes those rules in the menu bar and Settings.

Validation

xcodebuild build -scheme tabby -destination 'platform=macOS'
# ** BUILD SUCCEEDED **

xcodebuild test -scheme tabby -destination 'platform=macOS'
# Build completed, but the hosted macOS test bundle failed to load because of a local code-signing Team ID mismatch on tabbyTests.xctest.

Linked issues

Fixes #26

Risk / rollout notes

• Registrable-domain matching currently uses an in-repo normalizer with a documented compound-suffix table instead of a full public-suffix dependency. That keeps the implementation small and explicit, but uncommon suffixes may need follow-up entries if users report mismatches.
• Explicit domain Enable rules are persisted even when the parent browser app is disabled, so re-enabling the app later preserves the user’s per-domain intent instead of silently dropping it.