feat: add German, French and Russian translations

[lippdev]

What

• German (de-DE): full catalog contributed by a Reddit user (377 keys, placeholders checked against en-US).
• French (fr-FR) and Russian (ru-RU): new catalogs, machine-translated and not yet reviewed by native speakers. French uses "vous", Russian "вы".
• All three are in the language picker ("Deutsch", "Français", "Русский") and are matched from any regional Windows variant (de-AT, fr-CA, ru-KZ…).
• Installer: the three languages are in the language dialog (Inno's official German.isl, French.isl, Russian.isl), with translated custom messages and the registry value the app reads on first run. ConsoleMode.iss now has a UTF-8 BOM so Inno reads the Cyrillic text correctly.
• Tests: the "unknown language → English" examples moved from de-DE/fr-FR to ja-JP. New cases cover de-CH, fr-CA and ru-KZ.
• README (both), changelog (both) and manual checks in docs/TESTING.md.

Closes #14.

Testing

• dotnet test tests/ConsoleMode.Tests/ConsoleMode.Tests.csproj: 411 passed.
• dotnet build src/ConsoleMode (x64, Debug) on Windows: 0 errors.
• Ran the app in de/fr/ru: desktop Home and Settings, console Home, Session and System tabs. Nothing overflows. Known limit: on the console Session tab, the quick-setting tiles truncate the long "Resolution and refresh rate" and "Variable refresh rate (VRR)" captions with an ellipsis in fr/ru. Spanish already did this before, so it's a layout follow-up, not part of this PR.
• Compiled the installer with ISCC and opened it in Russian: the custom messages render correctly.

🤖 Generated with Claude Code

[pullfrog]

✅ No new issues found.

Reviewed changes: Reviewed the new German, French, and Russian localization catalogs and their integration into language selection, installer setup, tests, and user-facing documentation.

• Catalogs and language resolution: Added three complete catalogs and maps regional Windows language variants to the matching catalog while preserving English fallback.
• Installer integration: Added language-dialog options, translated custom messages, and registry values used by the app on first run; encoded the script as UTF-8 for Cyrillic text.
• Validation and documentation: Added catalog resources and tests for regional matching, key parity, and placeholders, plus README, changelog, and manual-checklist updates.

  ^{｜ View workflow run ｜ Using gpt-6-luna ｜ 𝕏}

[pullfrog]

✅ No new issues found.

Reviewed changes: Reviewed the delta since the prior Pullfrog review: the minimize-to-tray strings in the new language catalogs and the changelog context merged from main.

• Translated minimize-to-tray setting: Added the card title and description in German, French, and Russian, matching the English setting's behavior.
• Changelog context: Brought the upstream system-tray release note into both changelogs alongside the translation entry.

  ^{｜ View workflow run ｜ Using gpt-6-luna ｜ 𝕏}