feat(localization): add French localization support

[itishermann]

Description

Adds French (Français) as a selectable app UI language, introduces localized strings for all 280 keys including Romanize Lyrics and Enable %@ Scrobbling, and normalizes Localizable.xcstrings with Xcode-style spacing ("key" : value).

AI Prompt (Optional)

🤖 AI Prompt Used

- add French localization
- GeneralSettingsView: localize "Romanize Lyrics"
- ScrobblingSettingsView: localize dynamic "Enable \(service) Scrobbling"
- keep new strings in the catalog; align xcstrings with Xcode like spacing

AI Tool: Claude Code

Type of Change

• ✨ New feature (non-breaking change that adds functionality)
• 🎨 UI/UX improvement
• 🧪 Test update

Changes Made

• French (fr): fr localizations added across Localizable.xcstrings (alongside existing ar / ko / tr / id), full fr.lproj/Localizable.strings (280 entries), SettingsManager.ContentLanguage.french (languageCode: fr, picker label Français).
• Tests: AppLocalizationTests covers French Artist ("Artiste") and Subscribe %@ ("S'abonner") formatting.
• Romanize Lyrics: Catalog key + French translation ("Romaniser les paroles"), matching entry in fr.lproj.
• Enable %@ Scrobbling: French translation ("Activer le Scrobbling %@") in catalog and .lproj; %@ placeholder moves to end to match French grammar.
• Localizable.xcstrings: Normalized to Xcode-style JSON (space before :) after edits so future diffs are mostly substantive.

Testing

• Unit tests pass (swift test --skip KasetUITests — 1121 tests, all passing)
• Manual testing performed
• UI tested on macOS 26+

Checklist

• My code follows the project's style guidelines
• I have run swiftlint --strict && swiftformat .
• I have added tests that prove my fix/feature works
• New and existing unit tests pass locally
• I have checked for any performance implications
• My changes generate no new warnings (none introduced by these edits; existing project warnings may remain)

Additional Notes

• %@ in Enable %@ Scrobbling lets French place the service name (e.g. Last.fm) where grammar requires — here at the end: "Activer le Scrobbling %@".
• French uses « guillemets » for quotation marks in longer strings (e.g. the passkey note), following typographic convention.
• To reapply Xcode-style xcstrings after a tool emits compact JSON:

python3 -c "
import json, pathlib
p = pathlib.Path('Sources/Kaset/Resources/Localizable.xcstrings')
d = json.loads(p.read_text(encoding='utf-8'))
out = json.dumps(d, ensure_ascii=False, indent=2, separators=(', ', ' : '))
p.write_text('\n'.join(line.rstrip() for line in out.splitlines()) + '\n', encoding='utf-8')
"

[Copilot]

Pull request overview

Adds French UI localization support to Kaset, wiring a new selectable UI language into settings and providing French translations in the string catalog and generated .strings resources.

Changes:

• Add SettingsManager.ContentLanguage.french with display name and fr language code.
• Add French (fr) translations to Localizable.xcstrings and generate fr.lproj/Localizable.strings.
• Add unit tests verifying French bundle localization and formatted-string behavior.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
Tests/KasetTests/AppLocalizationTests.swift	Adds a French localization test covering a plain key (Artist) and formatted key (Subscribe %@).
Sources/Kaset/Services/SettingsManager.swift	Introduces ContentLanguage.french and maps it to language code fr for bundle lookup/locale.
Sources/Kaset/Resources/Localizable.xcstrings	Adds fr localizations (and normalizes formatting) including keys used by settings/UI.
Sources/Kaset/Resources/fr.lproj/Localizable.strings	Adds generated French strings resource file for runtime localization.

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