Fix #362: Spanish interface (es)

Adds a Spanish catalog (crates/ui-egui/src/i18n/es.tsv) covering the whole
interface like Japanese and Traditional Chinese: every menu string, every
tl! literal, the hint-bar fragments, plurals and the runtime strings the
other complete catalogs carry. It is written from the meaning of the
English labels in neutral, international Spanish (Latin American usage)
with the vocabulary users of vector illustration software know (Mesa de
trabajo, Muestras, Trazo, Relleno, Trazado, Pluma, Selección directa,
Buscatrazos, Motivo, Preajuste); no other application's localisation was
copied. Like Japanese it keeps the product, OpenType, the built-in
workspace names and the perspective grid preset names in English.

The language is registered with complete_menus (so the completeness
tests enforce it) and the one/other plural rule; every es-* system locale
(es-ES, es-MX, es-AR, es-419 ...) resolves to it.

Tests: es locale tags, Spanish vocabulary and plurals, Spanish letters in
the bundled UI fonts, and a new check that catalogs written with spaces
keep the leading/trailing spaces of joined fragments. ROADMAP, README and
docs/development.md list the language.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/development.md

@@ -76,7 +76,8 @@ always use the English ids and labels, so agents and scripts never see translate
 Languages shipped: English (`en`, the source), Traditional Chinese (`zh-hant`, complete, in the vocabulary used
 in Taiwan; `zh-TW`, `zh-HK`, `zh-MO` and `zh-Hant-*` locales all resolve to it), Simplified Chinese (`zh-hans`,
 complete, in the vocabulary used in mainland China; `zh-CN`, `zh-SG`, `zh-Hans-*` and a bare `zh` resolve to it,
-so the two scripts never mix), Japanese (`ja`, complete), Czech (`cs`, every menu label) and Brazilian Portuguese
+so the two scripts never mix), Japanese (`ja`, complete), Spanish (`es`, complete, in neutral
+international Spanish; every `es-*` locale such as `es-ES`, `es-MX`, `es-AR` or `es-419` resolves to it), 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