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

• Opened as draft so extension behavior and remaining Firefox lint warnings can be reviewed before release.
• 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 Firefox/Firefox for Android minimum-version compatibility warnings from preserving the Firefox 62 minimum with the new data-collection key, plus bundled dependency eval/innerHTML warnings.
• WXT reports existing React/MUI bundles over 500 kB after minification.

[rthaut]

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