i18n: add Ukrainian interface translation (#36)

Rebase onto main: the interface language is saved with main's other
preferences (cadcraft.prefs, beside the theme) instead of its own storage
key, on desktop and web; strings main added or changed in the touched
places (Dynamic Input settings, Properties header and current-property
rows, block picker, theme and command-history menu items, open/save
filters) are wrapped and translated; main's native-menu poll order is kept.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/localization.md

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+# Interface languages
+
+Choose **Edit → Interface Language** and select **Auto (system language)**, **English**, or
+**Українська**. The interface updates without restarting, including the native macOS menus.
+The choice is saved with the other interface preferences (`cadcraft.prefs`, beside the theme):
+eframe storage on the desktop, browser local storage on the web. Installations without a saved
+choice use Auto.
+
+Auto uses the first supported language in the operating system's preferred locales, or the
+browser's `navigator.languages` on the web. Ukrainian tags such as `uk-UA` and `uk_UA.UTF-8`
+resolve to `uk`; unsupported languages fall back to English. Set `CADCRAFT_LOCALE=uk-UA` to
+override the desktop's locale for a launch. An explicit saved English or Ukrainian choice
+takes precedence over that override.
+
+Automation can select the language through the existing control channel:
+
+```json
+{"id":1,"method":"engine.execute","params":{"command":"ui.language","params":{"lang":"uk"}}}
+```
+
+The `ui.set` method also accepts `{"interfaceLanguage":"uk"}`. Supported values are `auto`,
+`en`, and `uk`; invalid values are rejected without changing the current selection. Resetting
+palettes keeps the language choice.
+
+## Translation coverage
+
+Ukrainian covers command and menu labels, tool groups and tooltips, the Start page, file and
+model-space tabs, status controls, object snaps, drafting prompts, property labels, dialogs,
+and the About window's controls. The command reference can be searched by English command
+ID, English label, or Ukrainian label.
+
+Drawing titles, layer/layout/block/style names, parameter names and expressions, text in a
+drawing, and command-line input are user data and retain their original spelling. Clickable
+command keywords retain their English spelling so the text shown matches the input accepted
+by the engine. CLI/MCP/control responses, engine diagnostics and command history remain
+English. Uncatalogued text also falls back to English. Number formatting and the operating
+system's own file-picker controls follow the existing behavior.
+
+The bundled egui fonts cover Ukrainian, including `Ґґ Єє Іі Її`; the translation adds no font
+assets and works on the web without system fonts.
+
+## Adding translations
+
+The implementation follows the display-time catalog pattern used by
+[VectorCraft #205](https://github.com/storytold/vectorcraft/pull/205),
+[PdfCraft #106](https://github.com/storytold/pdfcraft/pull/106), and
+[FilmCraft #238](https://github.com/storytold/filmcraft/pull/238). Its code is local to CADCraft.
+
+- `crates/ui-egui/src/i18n/uk.tsv` is UTF-8 with three tab-separated columns:
+  `context`, `English source`, and `translation`. Ordinary labels have an empty context.
+  `@plural` entries hold Ukrainian `one|few|many` forms for integer counts.
+- Register a new catalog in `i18n::LANGUAGES`, add its preference variant and code/name,
+  and supply its integer plural rule if it uses `@plural` rows.
+- Use `crate::tl!("English label")` for a literal, `i18n::t(label)` for a known dynamic UI
+  label, and `crate::tf!("Version {version}", version = value)` for named placeholders.
+  Translate only at the display boundary; never look up arbitrary user names or identifiers.
+- Windows retain explicit IDs based on their English source, and tool groups retain their
+  source names as keys, so switching languages preserves their state.
+- Catalogs are embedded and parsed once. Missing or malformed rows cannot crash the app;
+  tests reject invalid rows, duplicate keys, changed placeholders, and mismatched ellipses.
+- `cargo test -p cadcraft-ui-egui i18n` checks menu/command/tool/snap coverage, every marked
+  UI literal, locale resolution, preferences, integer plurals, bundled glyphs and an actual
+  English/Ukrainian/English egui render. `cargo xtask ci` runs the workspace quality gates.
+
+Translations are original work under `MIT OR Apache-2.0`, written from the meaning of
+CADCraft's English strings. Do not copy proprietary localization resources.