i18n: add Polish (pl) interface language

Complete Polish catalog (4,492 rows): every menu string, tl! literal,
plural and status/error message, enforced by the existing completeness
tests (complete_menus, KEEPS_MENU_NAMES, COMPLETE_MESSAGES).

- pl.tsv: clean-room translation from the English meaning, following
  common Polish desktop-software conventions (obszar kompozycji for
  artboard, obszar roboczy for workspace, obrys, wypełnienie, próbki…)
- plural_polish: one / few (2–4, not 12–14) / many (0, 5–21, 25…)
- every pl-* locale (pl-PL, pl_PL.UTF-8) resolves to it
- tests: Polish plural forms, vocabulary, nested message translation;
  Polish letters in the bundled-font coverage test
- docs: README, development.md, localization-parity.md

Drafted with AI assistance; native review welcome.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DukHNvUY45rjRnvtz91x33

crates/ui-egui/src/dialogs/missing_fonts.rs

@@ -739,7 +739,7 @@ mod tests {
         let fr = crate::i18n::Lang::from_code("fr").unwrap();
         let shown = crate::i18n::message(fr, "2 font file(s) are now in /Fonts; 1 couldn't be added: a.otf: larger than 256 MB");
         assert!(shown.contains("se trouvent maintenant dans /Fonts") && shown.contains("a.otf") && shown.contains("plus de 256 Mo"), "{shown}");
-        for code in ["de", "es", "fr", "hu", "it", "ru"] {
+        for code in ["de", "es", "fr", "hu", "it", "pl", "ru"] {
             let lang = crate::i18n::Lang::from_code(code).unwrap();
             for s in [
                 "every search thread stopped on an internal error (please report this bug)",