Pending UI strings

A pull request that adds UI strings adds one new file here instead of editing assets/locales/<locale>.json. Two pull requests then never touch the same file, so they never conflict.

Name it after the change, lowercase with dashes: sketch-trim-hints.json.
Contents: a map from locale code to {"message id": "text"}. Give English; add other locales you can translate (a locale you leave out falls back to English, as before):
```
{
  "en": { "sketch.trim.hint": "Click a segment to trim it" },
  "de": { "sketch.trim.hint": "Segment zum Kürzen anklicken" }
}
```
The id must not already be in that locale's catalog, nor in another pending file; the build and cargo xtask lists fail otherwise. To change an existing message, edit the catalog itself.
Don't sort or fold anything. The app, cargo xtask i18n and the tests read these files as part of the catalogs, so your strings work in your own pull request.
After a merge, the Generated files workflow runs cargo xtask fold-pending on main: it adds the messages to the catalogs (sorted, one per line) and deletes the files. Never run it in a pull request.