[KnifeLemon]

What users notice

Settings → Theme has a new switch, Use the system font. When it's on, Filee draws its UI in the font the system uses, like other apps: Segoe UI Variable on Windows 11 (Segoe UI on Windows 10), San Francisco on macOS, fontconfig's default sans-serif on Linux. The switch applies right away. Filee's built-in Noto fonts stay behind the system fonts and still draw any script those lack. Han still follows the interface language (#1121): on Windows, a Japanese UI draws it in Yu Gothic UI, a Korean or English one in Malgun Gothic, Simplified Chinese in Microsoft YaHei and Traditional Chinese in Microsoft JhengHei. The switch is off by default, so nothing changes for anyone who doesn't turn it on.

What changed

• src/Filee.Core/Settings/AppSettings.cs: adds ThemeSettings.UseSystemFont, false by default. Settings saved before it existed read it as false, so no schema step is needed.
• src/Filee.App/Services/SoftPalette.cs: FontFamilies(language, systemFont). With the switch on, the platform's UI fonts go first, followed by the embedded chain, which stays unchanged. Only fonts without Han go ahead of the CJK fonts. On Linux, a fontconfig default that has Han is left out, so a CJK default can't give every language its Han shapes.
• src/Filee.App/SystemUiFontCollection.cs (new, registered in Program.cs and the test app): the system fonts under fonts:System#…, which match a letter only in the family asked for. This is needed because Avalonia's own system collection doesn't look in the requested family. It hands the letter to the platform's fallback, and on Windows that returns Yu Gothic UI for Han whatever family or language is asked. I checked it: "Segoe UI, Malgun Gothic" and even "Malgun Gothic" alone matched 直 in Yu Gothic UI. Without the wrapper, every language would draw Han in Japanese shapes. FileeFontCollection does the same for the embedded fonts ([Bug]: zh-CN UI renders some Han characters with Japanese/Korean glyph forms (bundled Noto Sans SC not used) #1095).
• ThemeService / AnnotationFonts: pass the switch through. Annotate's labels follow the same chain, because the text box they're typed in uses the UI font.
• ThemePage.axaml + ThemePageViewModel: the switch and its hint, in a card before "Reduce animations".
• i18n: theme.system_font and theme.system_font_hint in all 22 languages.
• docs/ARCHITECTURE.md: a short note on the font chain and why fonts:System exists.

Tests

• SystemFontTests (App): for every language, the embedded chain is the unchanged tail of the system chain. The first Han font is still the UI language's, for ja, ko, zh-CN, zh-TW, en and ar. On Windows, real lookups for A, م, न, ก and 直 at regular and semibold weight expect the first system font in the chain whose face has the letter, or the embedded font when none does, never the platform fallback. Each machine is asked for its own fonts: on the GitHub Windows Server images Segoe UI has no Arabic, so م correctly lands in Noto Sans Arabic there. Han must also land in a font of the UI language's region. The collection only matches the family it's asked for. The page switch saves and swaps AppFontFamily live.
• SystemFontSettingsTests (Core): off for new settings and for settings saved before this change, and kept once switched on.
• Run on Windows 11: SystemFontTests, LocalizationTests, LocalizationResourceTests, FileeFontCollectionTests, ThemePageTests, MaterialYouThemeTests: all pass. In the full Filee.App.Tests suite, 2266 passed, 33 were skipped and 2 failed, neither because of this change. RenderTests.Engines_page_shows_versions_descriptions_and_download_speed fails on this PC because it has its own Tesseract in Program Files, and an installed copy shows no version by design. DonutHighlightTests.A_lost_animation_frame_does_not_keep_the_slice_dark is a timing check that failed while the machine was busy; it passes when run on its own. dotnet format --verify-no-changes is clean on all four touched projects, and Filee.Setup still builds (it links SoftPalette).

Not covered

• macOS untested. The chain asks for .AppleSystemUIFont, then SF Pro Text and SF Pro. Whether Skia on macOS resolves the hidden system name needs a real Mac. If it doesn't, the chain falls through to the embedded fonts as before.
• Linux untested. fontconfig's default stands in for the desktop's UI font. GNOME and KDE can set their own UI font outside fontconfig's sans-serif, and that one isn't read.
• Found while testing, not changed here: the installer (SoftPalette.SystemFontFamilies) uses plain system names. By the lookup above, it probably draws Han in Yu Gothic UI in every language.

Closes #1065

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