feat(i18n): find UI text that is not marked for translation

npm run i18n now also lists, and fails on, markup text and title /
aria-label / placeholder without data-i18n*, and string literals that reach
the page (textContent, innerHTML, setAttribute, toasts, label:/title:
properties and similar) without t(), tn() or msg(). Opt-outs say why where
the text lives: translate="no" and data-i18n-runtime in markup,
// i18n-ignore in code. src/tools/ is skipped: its text is for AI clients.

534 places are reported today; npm test does not run this part yet.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/I18N.md

@@ -49,7 +49,7 @@ label.textContent = t(FEED_STATE_LABELS[state]);
 ## After you add or change text
 
 ```sh
-npm run i18n                     # shows what each language now lacks or no longer uses
+npm run i18n                     # what each language lacks or no longer uses, and unmarked UI text
 npm run i18n -- --missing ru     # the missing ru entries as JSON
 npm run i18n -- --prune          # drop entries whose English no longer exists
 ```
@@ -58,12 +58,29 @@ Changing the English changes the key, so the old translation goes unused and the
 
 To see a language, open the app with `?lang=ru`, or use the language switch in the Visual Presets dock.
 
+## Text the check reports as unmarked
+
+`npm run i18n` also lists UI text that is not marked at all, and fails while there is any:
+
+- **Markup:** element text without `data-i18n`, and `title`, `aria-label` or `placeholder` without the matching `data-i18n-*`.
+- **Code:** string literals that reach the page without `t()`, `tn()` or `msg()`: assigned to `textContent`, `innerText`, `innerHTML`, `title`, `placeholder`; passed to `setAttribute('title' | 'aria-label' | 'aria-valuetext' | 'placeholder' | 'alt', …)`, `createTextNode`, `insertAdjacentHTML`, `showToast`, `toast`, `setHint`, `announce`; or kept in `label:`, `title:`, `tooltip:`, `placeholder:`, `hint:`, `ariaLabel:`, `caption:`, `subtitle:`, `kicker:`, `emptyText:` properties.
+
+Fix it by marking the text. When the text must stay as it is, say so where it lives:
+
+| Case                                                                        | How                                                                      |
+| --------------------------------------------------------------------------- | ------------------------------------------------------------------------ |
+| Codes, units, product names in markup (`0.9 dB`, `ADS-B · 1090`, `OpenSky`) | `translate="no"` on the element; it covers everything inside, as in HTML |
+| Markup whose text code sets through `t()` (a status that starts as `OFF`)   | `data-i18n-runtime` on the element                                       |
+| A literal in code that is not UI copy (a product name, sample data)         | `// i18n-ignore: <reason>` on the line or the line above                 |
+
+Icons (`aria-hidden="true"`, Material Symbols), comments, CSS values and lowercase identifiers are skipped automatically, and so is `src/tools/`, whose text is written for AI clients rather than the page.
+
 ## Where things live
 
-| File                           | What it holds                                                                 |
-| ------------------------------ | ----------------------------------------------------------------------------- |
-| `src/i18n/index.js`            | `t`, `tn`, `msg`, the Intl formatters, markup translation, switching language |
-| `src/i18n/locale.js`           | shipped languages, their names, choosing the language at boot                 |
-| `src/i18n/locales/<code>.json` | one catalog per language, keyed by English text                               |
-| `scripts/i18n.mjs`             | `npm run i18n`: extracts messages and checks the catalogs                     |
-| `src/i18n/catalog.test.mjs`    | runs that check under `npm test`                                              |
+| File                           | What it holds                                                                  |
+| ------------------------------ | ------------------------------------------------------------------------------ |
+| `src/i18n/index.js`            | `t`, `tn`, `msg`, the Intl formatters, markup translation, switching language  |
+| `src/i18n/locale.js`           | shipped languages, their names, choosing the language at boot                  |
+| `src/i18n/locales/<code>.json` | one catalog per language, keyed by English text                                |
+| `scripts/i18n.mjs`             | `npm run i18n`: extracts messages, checks the catalogs, finds unmarked UI text |
+| `src/i18n/catalog.test.mjs`    | runs that check under `npm test`                                               |