M7: list the fonts a file uses that aren't installed (#752)

Text in a family that isn't installed was drawn in a fallback font
without a word. Now:

- `text.missingFonts` lists the families a document names that no
  installed or built-in font provides: text layers and their styled
  ranges, text styles, and string variables bound to a font family
  (every mode, through aliases), with the number of text layers.
- The desktop app shows a Missing fonts window after the user opens a
  file that has any: once installed fonts are indexed (before that every
  one of them would count as missing) and after any import report is
  closed. Files named on the command line count. Programmatic opens and
  the browser build never open it by themselves. Text > Missing fonts...
  shows it for the file in front; Escape closes it.
- `figcraft-cli info` prints `missing font` lines (`missingFonts` with
  --json).
- file-format-parity: the row claimed REST import warnings that no code
  produced.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/cli.md

@@ -29,7 +29,9 @@ be open to the public: [server.md](server.md#running-a-public-server)). See [ser
 ### `info <file> [--json]`
 
 Pages, node counts, top-level frames (id, type, name, size), style/variable/image counts. Repairs
-of a damaged file and import warnings go to stderr as `note:` lines (`notes` with `--json`).
+of a damaged file and import warnings go to stderr as `note:` lines (`notes` with `--json`). Fonts the
+file names that aren't installed are listed as `missing font` lines (`missingFonts: [{family,
+layers}]` with `--json`); their text renders in a fallback font.
 
 Every subcommand that reads a file also takes the public REST API file JSON (the response of
 `GET /v1/files/:key` or of its nodes endpoint, written to a `.json` file): it is imported on the fly.