M7: localization layer (#128)

UI strings come from per-language TOML catalogs compiled in (English text as the key and fallback, named placeholders, one/other plurals, context suffixes); command labels are translated at display time while ids, CLI, MCP and control output stay English; accessibility names follow. Preferences > Language (System / English / Español, live; the system language from the OS or the browser), pref.language for scripts. A key check test guards the catalogs. The Spanish catalog is a machine draft awaiting native review; docs/translating.md explains adding a language.

docs/translating.md

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+# Translating FigCraft
+
+FigCraft's interface strings live in one TOML catalog per language in
+[`crates/ui-egui/locales/`](../crates/ui-egui/locales). The build bakes every catalog into the
+binary (desktop and web); nothing is read at run time. Preferences ▸ Language picks the language
+(System follows the OS, or the browser on the web) and the interface switches at once. `pref.language
+{language}` does the same from the CLI, MCP and the control channel.
+
+## The catalogs
+
+- **`en.toml`** lists every key. A key is the English text as the code writes it, so the code stays
+  readable: `tr("Fill")`, `trf("Delete {name}?", &[("name", &name)])`, `trn("{n} layers", count, &[])`.
+  Values equal their keys except plurals, which give both forms:
+  `"{n} layers" = { one = "{n} layer", other = "{n} layers" }`.
+- **`es.toml`** and other languages translate the same keys under `[ui]`, plus command labels by
+  command id under `[commands]` (the Actions palette, the shortcuts panel and the menus) and, under
+  `[menu]`, the few menu items the main menu words differently from the command (the Select matching
+  submenu's "Fill" for "Select same fill"). Command ids, the CLI, MCP and control-channel output stay
+  English.
+- `[meta] name` is the language's own name (the picker shows it untranslated); `[meta] status` marks
+  a draft (the Preferences window says so).
+
+A key a catalog lacks shows its English text, so a partial catalog is safe to ship.
+
+## Adding a language
+
+1. Copy `crates/ui-egui/locales/es.toml` to `<code>.toml` (a BCP 47 code: `fr`, `pt-BR`, `ja`) and
+   set `[meta] name` and `status = "draft — needs native review"`.
+2. Translate every value; keep the keys. Keep `{placeholders}` exactly, "…" where English has it,
+   brand names and keyboard glyphs. English labels of commands: `figcraft-cli commands`.
+3. Plurals: give `one` only if the language has a singular form (Japanese, Korean and Chinese give
+   only `other`). `trn` picks `one` for a count of 1; a language with other rules (French: 0 and 1)
+   needs its rule in `plural_one` in [`crates/ui-egui/src/i18n.rs`](../crates/ui-egui/src/i18n.rs).
+4. To require full coverage, add the code to `COMPLETE` in
+   [`crates/ui-egui/src/i18n_tests.rs`](../crates/ui-egui/src/i18n_tests.rs).
+5. Run the check and look at the panels in the app (longer words must fit the right panel):
+
+       cargo test -p figcraft-ui-egui i18n
+
+The check fails on keys the code uses that `en.toml` lacks (it prints the lines to paste), keys no
+code uses, unknown command ids, placeholders that don't match English, and, for complete languages,
+anything untranslated. Machine-drafted catalogs stay marked as drafts until a native speaker has
+reviewed them.
+
+## Adding strings in code
+
+Wrap every string a user sees or a screen reader hears: `tr` for plain text, `trf` for `{named}`
+placeholders, `trn` for counts, `tr_noop` for literals in `const` tables that are looked up later
+with `tr(label)`. The key must be a string literal as the first argument (the check reads the
+source). Don't wrap ids, command ids, JSON, log messages, file extensions or document content. Then
+add the key to `en.toml` and to each complete language.
+
+Labels from the engine and document crates (tool names, blend modes, easing curves…) are looked up
+at display with `tr(x.label())`; they are listed in `dynamic_labels` in `i18n_tests.rs`.