i18n: Extensions panel (Gallery / Installed / Diagnostics tabs) — the entire diagnostics UI set is hardcoded English

The Extensions settings pane (a fairly visible surface now that it has Gallery/Installed/Diagnostics tabs) is almost entirely hardcoded English in static/index.html + static/panels.js. Same pattern as the other recently localized sections but with no keys at all.

These use plain HTML text in index.html (no data-i18n attribute) and template literals in panels.js without t():

• Browse/install/manage meta text and the sub-tab loading placeholders
• The whole «Trust model» explainer block
• 14+ diagnostic summary labels (Extension dir configured / valid, Manifest configured / loaded / status / counts, etc.)
• Boolean badges (true / false), Enabled/Disabled badge, None, No warnings
• Sidecar card labels (Origin, Health path, Health URL, Proxy, Proxy path) and the consent buttons («Approve proxy consent» / «Revoke proxy consent»)
• «Final public asset URLs» / «Scripts» / «Stylesheets» / «Sanitized warnings» cards and their meta descriptions
• extension_state_unknown_ids_hint full sentence warning

Happy to send a PR adding keys + wiring the call sites; local translations already exist (a similar pass was done for the rest of settings).