i18n (cs): translate labels that bypass the catalog lookup (file filters, diagnostics, credits, gradients, print marks)

[Losminek]

Follow-up to #1189 and #1226 (Czech catalog). The catalog is complete, but a few labels never reach the
catalog lookup, so they still show in English under Czech. This PR translates them where they are shown,
from a variable or a template, so every other language keeps its current behaviour and falls back to
English. No new tl!("literal") keys, so only cs.tsv gains rows.

What changes

• File dialog filters (desktop apps/vectorcraft/src/main.rs, web apps/vectorcraft-web/src/web.rs):
  filter names such as "All readable files", "Swatch libraries", "PDF presets" are looked up when the
  dialog is built (context file-filter). Extensions and format names stay as they are.
• Diagnostics line (chrome.rs): the words render / ui are looked up (context diagnostics);
  numbers, ms and fps stay.
• New Document: the suggested name is the template Preset {number} instead of format!("Preset {}").
• Credits: the PRs column header and the contributor tooltip go through named placeholders, with
  @plural rows for PR and commit counts. Names and handles stay unchanged.
• Gradients: Czech needs the adjective before the noun and in agreement, so {kind} Gradient now reads
  "Lineární přechod" / "Kruhový přechod"; Radial got a gradient-kind context for the masculine form.
• "Japanese" collision: the English key is both the language name and a printer mark style in PDF/Print.
  The mark style now uses a mark-style context ("Japonské"), so the language stays "Japonština". English
  output and the stored id are unchanged.
• Regression tests for the file filters, credits plurals, gradient wording and the mark-style context;
  docs/localization-parity.md, docs/gaps.md, docs/roadmap.md, docs/target-app-parity.md and
  ROADMAP.md note the change (scores unchanged; the stale "Czech (menus only)" line is corrected).

I also checked the search box placeholder, Home screen, status bar, toolbar and panel tooltips and the
Preferences window: they already use lookups, so nothing else needed changing there. Workspace names stay
English on purpose (MENU_KEEP_AS_IS).

Checks

Clean environment, --locked: cargo fmt --all -- --check clean; cargo test -p vectorcraft-ui-egui:
1,043 passed, 0 failed (including the bundled-font glyph test); cargo clippy -p vectorcraft-ui-egui --all-targets -- -D warnings clean; desktop and wasm cargo check OK.

Review

The 19 new Czech rows were drafted with AI assistance (Codex and Claude) and checked against the existing
catalog terminology; they sit under a comment in cs.tsv and have not yet been proofread by a native speaker. Wording worth a second look: "Všechny čitelné soubory" (All readable
files), "Všechny soubory pro umístění" (All placeable files) and the contributor tooltip.

🤖 Generated with Claude Code

[bflatastic]

Thanks @Losminek! The Czech labels that bypassed the catalog now show in Czech.

I rebased it onto main and merged it after the full local gate passed on the combined branch. The rebase had conflicts with tonight's other merges: cs.tsv and tests_modal.rs (both sides kept), and the docs. In docs/roadmap.md (M14 row) and docs/target-app-parity.md (UI chrome row) I kept main's newer rows, so the Czech note in those two rows didn't make it; it is still in docs/localization-parity.md and docs/gaps.md. Live check on Windows: in Czech the status bar reads "vykreslení … · rozhraní …".