feat: add "Windowed fullscreen" toggle for borderless on-Space mode

[koto9x]

Summary

Adds a user-toggleable Windowed fullscreen mode to the Fullscreen view. When enabled, the fullscreen window opens as a movable/resizable borderless-style window pinned to the current Space's visible frame, instead of macOS native fullscreen — which animates to a new Space and hides the menubar and Dock.

Default is off, so existing behavior is preserved 1:1 for current users.

Why

Native fullscreen forcing a new Space breaks common multi-window workflows — lyrics-while-coding, lyrics-while-browsing, lyrics on a second display while the primary stays free. "Windowed fullscreen" is the same idiom most games offer: visually fills the screen, but stays on the current Space and can be moved/resized like any other window.

What changed

UserDefaultStorage.swift — adds useWindowedFullscreen: Bool = false (macOS-only, persisted via the existing @ObservableUserDefault pattern).

LyricFever.swift — the Fullscreen Window's onAppear now branches on the flag:

• Off (default, unchanged): collectionBehavior = .fullScreenPrimary + the existing toggleFullScreen(true) path.
• On: borderless styleMask (.titled, .closable, .miniaturizable, .resizable, .fullSizeContentView) with titlebarAppearsTransparent + titleVisibility = .hidden, collectionBehavior = [.managed], setFrame(screen.visibleFrame, display: true), isMovableByWindowBackground = true. If macOS restored prior native-fullscreen state, exits it first via toggleFullScreen(nil) so the styleMask change can actually land; sets isRestorable = false so subsequent launches start clean.

.windowFullScreenBehavior is now bound to the flag (.disabled when on, .enabled when off), so the green traffic-light button can't pull a windowed session back into native fullscreen.

FullscreenView.swift — new toggle in the existing in-fullscreen settings popover (next to Blur surrounding lyrics and Animate on startup):

Toggle(\"Windowed fullscreen (stays on current Space)\", isOn: \$viewmodel.userDefaultStorage.useWindowedFullscreen)

Drive-by: responsive album art

The album-art frame in FullscreenView was hard-coded at 550pt / 700pt. That looks fine at 1920×1080 native fullscreen, but in windowed mode the user can resize the window and the fixed-size art overflows the lyrics column. Changed to derive from the GeometryReader container:

let columnWidth = viewmodel.canDisplayLyrics ? containerSize.width * 0.5 : containerSize.width
let artSide = max(120, min(columnWidth * 0.85, containerSize.height * 0.55))

Min 120pt so it doesn't disappear at tiny sizes. On a typical native-fullscreen display the resulting size matches the previous hard-codes within ~5%, so the existing experience is effectively unchanged.

Test plan

• Builds against main (xcodebuild Debug, -skipMacroValidation).
• Default off: Fullscreen opens via native fullscreen as before, animates to a new Space, green button toggles, Esc blocked.
• Toggle on, reopen Fullscreen: opens as a borderless window at visible-frame size on the current Space. Can drag from the window background; can resize from edges/corners; menubar and Dock remain visible.
• Album art resizes smoothly when the windowed-fullscreen window is dragged smaller/larger; lyrics column no longer overflows.
• Toggling the setting back off and reopening Fullscreen returns to native-fullscreen behavior.

Notes

• The Apple Music workaround / MediaController(bundleIdentifier:) change on main is independent of this PR — same compile state as base.
• No new dependencies. No localization strings added beyond the one toggle label (happy to wire into Localizable.xcstrings if preferred).