Lay the theme-relative foundation: tokens, control themes, icons, system font

[chrisuthe]

Phase 1 of the UI reskin: everything visual expressed once as resources, so the later phases only lay controls out. The shell belongs to the OS — native decorations, the system light/dark variant and accent followed live, the platform's UI font — and the content area belongs to the music. The layout inside the window will come from Sendspin for Windows; the colours do not. No layout changes here.

What changed

• Tokens (Styles/Tokens.axaml): VeilBrush, TranslucentSurfaceBrush, ArtPlaceholderBrush, GlowDefaultBrush in Light and Dark dictionaries, plus OnAccentBrush. Every colour is a DynamicResource into Fluent, so nothing is a literal. Fluent in Avalonia 12.1.1 turned out to carry only the SystemControl* resource set; the SystemFillColorCautionBrush the old warning class named did not exist, so it now uses Fluent's error-text brush.
• Control themes and text (Styles/PlayerStyles.axaml): a PathIcon theme that strokes at 1.7 units on a 24 box; iconButton, playButton, primary; one text scale with the old class names as aliases on the same selectors.
• Icons (Styles/Icons.axaml): shuffle, repeat and switch-group from Sendspin for Windows, the rest from Feather (both MIT, named in the header). Every emoji glyph in the views is gone; play/pause is an icon choice on IsPlaying, repeat shows repeat-one when the server says so.
• Accent: never overridden. OnAccentBrush is black or white from the accent's WCAG luminance (Core/Theme/AccentContrast.cs), recomputed behind one deduplicated subscription to the platform's colour-changed event (SystemColorChangeFilter, for the ~20-event storm measured on Windows). The threshold is set between the saturated picks Windows and GNOME serve and the light highlights Plasma serves.
• System font: Fluent's font resource is overridden to $Default, Inter stays as glyph fallback. On Linux the desktop font comes from the Settings portal's font-name key, which the KDE backend serves too — and it is reachable from the Flatpak, where fontconfig's own default is DejaVu Sans; measured in the runtime sandbox. Windows measured plain Segoe UI and stays unset; macOS measured Helvetica, and the SF override is a follow-up once the resolvable family name is known. The app now logs what it resolved at start-up.
• Icons: the green disc from the reference app at 256 px for the app, tray, AppImage and Flatpak.
• Docs: README, App.axaml, MainWindow.axaml and the styles header state the principle; the "not copied" wording is gone. docs/ARCHITECTURE.md records the font decision and the Windows and macOS measurements.

Before / after

	Light	Dark
Before (master)
After

Both taken on the Wayland head under Plasma 6.7, switching live with plasma-apply-colorscheme. Start-up log on this box:

UI font: $Default is Noto Sans, glyphs from Noto Sans; controls use $Default, glyphs from Noto Sans; fallback face Inter

Checks

• make test green in Debug and Release (258 + 22).
• dotnet format --verify-no-changes clean on every project touched. One whitespace-only commit re-indents the MPRIS GetAll block that dotnet format already objected to on master.
• Every Linux-buildable project builds clean in Release with warnings as errors.
• Tests pin: every token resolves under both variants; every icon is a geometry inside the 24 box; no axaml names a colour and no view draws a glyph with a character; the on-accent brush; the luminance helper; the font-name parser; the change filter.

Left for later phases

• The header's Diagnostics and Settings toggles keep Fluent's white-on-accent checked look until Phase 2/3 rebuilds the header.
• The macOS DefaultFamilyName for SF, once the resolvable name is measured.