[joske]

On Windows, yserver has a Settings dialog, like the macOS one, and uses its own icon instead of the generic Windows one.

• Opening it: from the tray icon, which now appears in rooted mode too, or from the rooted window's title-bar menu (right-click). The tray menu has Settings… and Exit.
• The settings:
  • the X display, with a Copy button;
  • rootless or one window for the X screen;
  • clipboard sync;
  • start yserver when you sign in (the per-user Run registry key).
• Applying them:
  • Clipboard sync applies at once. The mode applies when you restart, through the dialog's Restart button.
  • Options given on the command line are shown greyed out.
  • Restart relaunches with the same arguments, so -auth is kept.
• Default mode: saved settings apply at every start, so plain yserver.exe now starts rootless by default, as on macOS. --rooted and --rootless override that for one run.
• Where they're saved: %APPDATA%\yserver\settings.conf.
• Not on Windows: Scale (yserver isn't DPI-aware yet), and the Mac-only button emulation and key equivalents.
• Keys: in rooted mode, Alt+Space, F10 and a lone Alt go to X clients, not to the Windows window menu, so an X window manager's own Alt+Space menu still works.
• The icon: assets/yserver.ico, made from the existing SVG, is embedded in yserver.exe, so Explorer and the taskbar show it. The tray and both window types use it.

Verification:

• Unit tests for the settings file format and for how the Run key quotes its command line. The quoting test vectors were checked against Windows' own CommandLineToArgvW.
• In a Windows 11 VM, rooted and rootless:
  • the dialog opens from the tray and from the title-bar menu;
  • Copy puts the display on the clipboard;
  • with clipboard sync off, copy and paste stops crossing in both directions, and resumes when it's turned back on;
  • start at sign-in writes the Run value and removes it again;
  • the chosen mode survives a restart;
  • xev receives Alt+Space, F10 and a lone Alt.
• Builds: Linux formatting, clippy and tests pass, and Windows clippy is clean. This PR's CI has no Windows job yet; the next PR adds one and builds this code.

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