i18n: Czech interface language

[petrleocompel]

Adds Czech (Čeština, cs) as a third interface language next to English and Japanese, building on the current i18n.rs design.

What's included

• The language itself: Language::Cs (serialised as "cs") is listed under VectorCraft ▸ Language as "Čeština" and is accepted by app.language {lang: en|ja|cs}. It is saved and restored like the other languages. tr() now picks the table per language with a match and cannot panic.
• crates/ui-egui/src/i18n/czech.rs: 556 entries in menu order.
  • They translate every menu label (539 of 561). The 22 labels kept as they are form an explicit KEEP_AS_IS list: the product name, language names, OpenType, the built-in workspace names and the perspective-grid preset names.
  • They also translate the 17 labels that only appear after a toggle is flipped (Show/Hide Guides, Lock/Unlock Grid, Outline/Preview, Edit Contents/Edit Envelope…), so the menu stays Czech after you click one.
• Tests:
  • Czech covers every menu label except fonts, sizes, plug-in names and KEEP_AS_IS. Toggle labels are collected by flipping each toggle on a real document.
  • Every Japanese and Czech entry is a real menu label; entries are unique; a translation ends in … exactly when the English does.
  • cs is accepted, saved and restored, and the Language menu lists every language.
  • czech_glyphs_are_available_without_system_fonts checks that the primary UI fonts (Source Sans 3, JetBrains Mono and egui's built-in fallbacks) contain áčďéěíňóřšťúůýž, their capitals and „“‚‘…–.
• dialogs/plugin.rs: the "No filter plug-ins installed" header is now a pub const. A test running in parallel can install plug-ins and remove that header, so the label test refers to the constant to stay reliable.
• ROADMAP.md: the interface-language line mentions Czech.

Translation notes

• Written from the meaning of the English labels using standard Czech desktop conventions: menus are nouns (Soubor, Úpravy, Objekt, Text, Výběr, Efekt, Zobrazení, Okno, Nápověda) and commands are infinitives (Otevřít…, Uložit jako…, Seskupit, Oddělit). This is clean-room work: no other application's Czech localisation was consulted.
• Some key terms:
  • artboard → pracovní plocha
  • Pathfinder → Operace s cestami
  • Expand → Rozložit
  • Swatches → Vzorník
  • Pattern → Vzorek
  • Blend (object) → Prolnutí
  • Blending Mode → Režim míchání
  • Envelope Distort → Deformace obálkou
  • Create Outlines → Převést na křivky
  • Image Trace → Vektorizace obrazu
• Workspace names stay in English because the workspace switcher and saved preferences show them untranslated.
• Not translated, as on main: generated labels ("Undo Move", "Last Effect: X") and the command palette, which doesn't call tr().

Testing

• cargo test -p vectorcraft-ui-egui: 516 passed (7 of them i18n tests).
• cargo fmt --all --check passes.
• cargo clippy -p vectorcraft-ui-egui -p vectorcraft --all-targets -- -D warnings passes.
• cargo check -p vectorcraft passes.
• cargo xtask layers passes.
• The wasm check was not run locally (the target isn't installed).

🤖 Generated with Claude Code

[echelon]

Děkujeme, @petrleocompel, merged! A complete Czech interface (539 of 561 menu labels, plus the labels that only show after a toggle is flipped) is great, and the tests are excellent: coverage of every menu label with an explicit KEEP_AS_IS list, the ellipsis check, real menu labels only, and glyph coverage in the bundled UI fonts. I merged main into your branch after the vertical-type PRs landed (one ROADMAP line now covers Japanese, Czech and vertical type, and notes that Japanese glyphs come from craft-fonts). Full CI passes.

Heads-up: #205 proposes a TSV-catalog i18n framework. If it lands, Czech would move into it (we've asked that PR to carry your table over).

If you're not already there, come say hi in our Discord, we'd love to have you: https://discord.gg/artcraft