Migrate extension build to WXT

[rthaut]

Summary

• Replace WebExtension Toolbox build tooling with WXT MV2 builds for Chrome, Edge, and Firefox.
• Move entrypoints, static assets, icons, and locales into WXT's expected layout while preserving MV2 page_action behavior.
• Add manifest smoke checks and update CI to run lint, all browser builds, all browser zip packages, and Firefox extension linting.

Validation

• npm ci
• npm run lint
• npm run test:builds
• npm run test:packages
• npm run test:firefox:lint
• npm test

Notes

• Firefox manifest now declares browser_specific_settings.gecko.data_collection_permissions.required as ["none"] because the extension does not collect or transmit data outside the extension.
• Firefox lint passes with 0 errors and 6 warnings:
  • Two minimum-version compatibility warnings (KEY_FIREFOX_UNSUPPORTED_BY_MIN_VERSION / KEY_FIREFOX_ANDROID_UNSUPPORTED_BY_MIN_VERSION) because data_collection_permissions support was introduced in Firefox 140 (desktop) / 142 (Android) while this extension keeps strict_min_version: "62.0". These warnings are expected and accepted: the key is simply ignored by Firefox versions older than 140, and the Firefox 62 minimum is kept intentionally so users on older Firefox-based browsers can continue to install the extension.
  • Bundled dependency eval/innerHTML warnings, pre-existing bundle noise unchanged by this migration.
• WXT reports existing React/MUI bundles over 500 kB after minification.
• Store releases of the WXT-built packages will follow separately after the usual release process.

[rthaut]

Visual recap: https://plan.agent-native.com/_agent-native/open?app=plan&view=plan&to=%2Frecaps%2Frecap-8d226155af6e47b1&planId=recap-8d226155af6e47b1&agentSidebar=closed