[Agent] i18n: convert remaining hardcoded UI strings and add a no-literal-string guard #655

Problem and evidence

A first pass (commit f0f19245) moved about 120 hardcoded English strings to t() keys: the building plan, the setup wizard, and the specific titles and labels a frontend audit listed. Several surfaces are still mostly literal English:

• web/js/components/preact/UI.jsx: the recording video modal, delete and snapshot dialogs, and fullscreen errors. Only the strings the audit listed were converted.
• web/js/components/preact/timeline/CalendarPicker.jsx: month and day names and navigation labels.
• FisheyeEptzCanvas.jsx: aria and title strings (partly done).
• Anything else a sweep finds: JSX text nodes and title= / aria-label= / placeholder= literals across web/js/components/preact/**.

Expected behavior and scope

1. Sweep web/js for user-visible literals and convert them to t('<namespace>.<key>'), adding the keys to web/public/locales/en.json only. Other locales fall back to English through web/js/i18n.js. Translations are tracked in a separate issue.
2. Dates, months and weekdays should use Intl.DateTimeFormat with the active locale instead of translation keys where that's possible (CalendarPicker).
3. Add a guard so the regression doesn't come back. For example, an ESLint rule such as eslint-plugin-i18next's no-literal-string in markup-only mode, scoped to web/js/components/** with an allowlist for technical strings. If the rule is too noisy, a Jest test that scans the JSX for >[A-Z][a-z]+ text nodes is acceptable.
4. Don't translate log messages, console.* output, CSS class names or API paths.

Acceptance criteria

• No user-visible English literals remain in UI.jsx or CalendarPicker.jsx.
• The lint or test guard runs in CI and fails on a newly added literal.
• en.json keys are named consistently with existing namespaces (recordings.*, timeline.*, live.*, etc.), and keys that already exist are reused, not duplicated.
• t() is only called where it's in scope. Watch module-level helpers that have no t, and run eslint --rule 'no-undef: error' on touched files.

Validation

• cd web && npx vite build (this is the JSX compile check; npx babel gives false errors), npm run lint, npm test.
• Spot-check the UI with the language set to es: converted strings show English (fallback) without raw key names.

Constraints and rollback

• Mechanical refactor. Keep PRs reviewable; splitting by directory is fine.
• Rollback: revert.