Add fullscreen compatibility mode

[PathGao]

Summary

Adds an Advanced setting for users who need native macOS / WebKit fullscreen for YouTube and Bilibili. The setting is applied on the next launch because WebKit chooses its fullscreen host while creating the web view.

• Wallpaper mode remains the default: YouTube gets a runtime-only fs=0 and keeps Nifro’s wallpaper presentation.
• Compatibility mode starts as a regular app, enables WKPreferences.isElementFullscreenEnabled, and removes every stale fs parameter from the runtime YouTube player URL.
• YouTube is loaded directly with Nifro’s Referer header, rather than through an app-built iframe, so element fullscreen is requested by the actual WKWebView document.
• Saved website URLs are not mutated; presentation policy is derived at load time.

Scope

Does not include the macOS Control Center menu-bar permission repair from #124 or any machine-local Control Center state.

Verification

• swift test — 219 tests / 49 suites passed
• xcodebuild -project Nifro.xcodeproj -scheme Nifro -configuration Release -derivedDataPath .build/pr-derived-2 CODE_SIGNING_ALLOWED=NO build
• jq empty Nifro/Localizable.xcstrings
• git diff --check