Add Finnish interface and date language

Adds a Finnish (Suomi, `fi`) catalog with 2,592 entries: every key of
the other bundled catalogs, written in sentence case from the English
labels. Finnish uses the one/other plural rule with the partitive
singular after any number but 1 (1 sivu, 2 sivua, 0 sivua).

Finnish is also a date language: calendar headings use the nominative
month (Lokakuu 2026) and dates the partitive (14. lokakuuta 2026).
Any fi-* locale selects it.

Tests cover registration, locale tags, persistence, plurals, calendar
wording, commands/tools, scanned UI literals, every bundled catalog key
and the Latin UI fonts. The catalog has its ATTRIBUTION.toml entry
(ATTRIBUTION.md regenerated with `cargo xtask assets --write`).

Clean-room, AI-assisted (Claude Code) translation; not yet proofread
line by line by a native speaker.

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

ATTRIBUTION.toml

@@ -231,6 +231,17 @@ usage = "Lithuanian UI strings, compiled in by crates/ui-egui/src/i18n/mod.rs"
 sha256 = "7fa70f68a9fa24d0b1cac05fe3aaabaeaca9750c57416124bd317a4eda806d5a"
 
 
+[[asset]]
+path = "crates/ui-egui/src/i18n/fi.tsv"
+title = "Finnish interface translation catalog"
+author = "PdfCraft contributors"
+source = "Contributor-original: AI-assisted (Claude Code) clean-room translations of PdfCraft's English UI labels and user-facing messages; not yet proofread by a native Finnish speaker"
+licence = "MIT OR Apache-2.0"
+licence_file = "LICENSE-MIT"
+kind = "translation"
+usage = "Finnish UI strings, compiled in by crates/ui-egui/src/i18n/mod.rs"
+sha256 = "01e5422ceb5fb7dd8bd23d6da93ce5210357589ceec6e4cf1725c56da13d1a98"
+
 [[asset]]
 path = "assets/icons/accessibility.svg"
 title = "Lucide icon \"accessibility\""