i18n: add Hungarian (hu) UI translation

Add the Hungarian catalog (4,524 rows): every menu string, every `tl!`
literal, the contextual and `@id` rows, the plurals and the status and
error messages, written from the meaning of the English labels in
general Hungarian vector-illustration vocabulary. Conventions are in the
header of `hu.tsv`.

- Register `hu` ("Magyar") in `i18n::LANGUAGES` as a complete language
  with `plural_none` (a count takes the singular); every `hu-*` locale
  resolves to it.
- Tests: Hungarian joins the menu-label, menu-string, `tl!`-literal and
  message coverage checks; `hungarian_reads_as_hungarian` pins the core
  vocabulary, the plural, the `file.close` override and a nested
  message; ő ű Ő Ű are checked in the bundled UI fonts.
- README, ROADMAP, docs/development.md and docs/localization-parity.md
  list Hungarian.

docs/development.md

@@ -177,7 +177,8 @@ international Spanish; every `es-*` locale such as `es-ES`, `es-MX`, `es-AR` or
 complete; `fr-FR`, `fr-BE`, `fr-CA`, `fr-CH` and every other `fr-*` locale resolve to it), Italian (`it`,
 complete; `it-IT`, `it-CH` and every other `it-*` locale resolve to it), Russian (`ru`, complete; every `ru-*`
 locale such as `ru-RU`, `ru-BY` or `ru-KZ` resolves to it), Ukrainian (`uk`, complete; `uk-UA`,
-`uk_UA.UTF-8` and every other `uk-*` locale resolve to it; integer plurals use one/few/many), Czech (`cs`, every menu label) and Brazilian Portuguese
+`uk_UA.UTF-8` and every other `uk-*` locale resolve to it; integer plurals use one/few/many), Hungarian (`hu`, complete;
+`hu-HU` and every other `hu-*` locale resolve to it; a count takes the singular, so plurals have one form), Czech (`cs`, every menu label) and Brazilian Portuguese
 (`pt-br`, every menu label and every `tl!` literal). Untranslated text falls back to English until its rows are added.
 
 - `tl!("…")` translates a literal into the language the UI is drawn in; `i18n::t(s)` is the same for a
@@ -205,10 +206,10 @@ locale such as `ru-RU`, `ru-BY` or `ru-KZ` resolves to it), Ukrainian (`uk`, com
   control channel, MCP and tests read them) and are translated only where the status bar draws them
   (`i18n::msg`). `@msg` catalog rows hold a whole message or a template such as
   `Couldn't open {name}: {e}`; `{_1}`, `{_2}` … stand for the format string's `{}`, and the values in the
-  placeholders are translated in turn (the reason after `: {e}` is often a message too). Spanish, French, Italian, Japanese, Russian and Ukrainian cover every
+  placeholders are translated in turn (the reason after `: {e}` is often a message too). Spanish, French, Hungarian, Italian, Japanese, Russian and Ukrainian cover every
   message literal the test scan finds (`complete_languages_translate_every_message`, languages listed in
   `COMPLETE_MESSAGES`): a new `Err("…")`, `Other(…)`, `#[error(…)]` or `status(…)` message needs an `es.tsv`,
-  a `fr.tsv`, an `it.tsv`, a `ja.tsv`, a `ru.tsv` and a `uk.tsv` row (`VECTORCRAFT_I18N_DUMP_MESSAGES=messages.txt cargo test -p vectorcraft-ui-egui
+  a `fr.tsv`, a `hu.tsv`, an `it.tsv`, a `ja.tsv`, a `ru.tsv` and a `uk.tsv` row (`VECTORCRAFT_I18N_DUMP_MESSAGES=messages.txt cargo test -p vectorcraft-ui-egui
   complete_languages_translate_every_message` lists them all). Other languages show messages in English
   until they add `@msg` rows.
 - Not translated on purpose: names that are user data (layers, swatches, fonts, documents), the tab