Adds a bundled add-on `translate` with a sidebar panel + a right-click
"Translate selection" menu item. Two swappable backends:
- LibreTranslate (default) — free MIT engine; the panel's Settings tab
lets the user point at any instance (public or self-hosted) and drop
in an API key if one's required.
- Google (unofficial free endpoint at translate.googleapis.com/
translate_a/single) — no key, wide coverage, but unofficial and
Google can break it any time. Opt-in fallback.
Flow: user selects text on a page, right-clicks -> "Translate
selection". Add-on's context-menu handler stashes the selection under
storage.__pending and calls api.revealSidebar("main"); the panel
loads, drains __pending on first paint, and translates. Ctrl/Cmd+Enter
in the input textarea also translates. Source + target language
choices, browser-language default target, swap button, copy-to-
clipboard on the output, settings gear.
Depends on a new "context-menu-item" capability + api.revealSidebar
hook in addons-host.js / main.js. Those wiring changes are prepared
but not committed here — a parallel session is refactoring the same
functions concurrently, so the safe path is to land translate/ first
and let the wiring go in alongside the next host-facing commit. Until
the wiring lands, the manifest's "context-menu-item" cap is silently
dropped (per validateManifest's unknown-caps policy) and the sidebar
panel + the panel's translation UI still work standalone — the
right-click entry point is what's gated.
