WebView: revert background-colour patches (experimental)

[jakepenn]

Summary

Reverts the four Minimal patches that customise WebView background painting on macOS/iOS and Windows. Purpose: validate whether the white-flash fixes were actually necessary, and isolate them as the cause of monitor-dependent pixelation we're now seeing in the Memory Rites player on macOS (high-DPI WKWebView content rasterising at 1x when the host window moves between displays with different + "backingScaleFactor" + ).

Reverts, newest first:

• + "a8b2868" + — WebView (Windows): suppress background flash on creation and hide/show
• + "fe88d6d" + — WebView (Windows): pre-seed renderer background colour via CLI switch
• + "5575dec" + — WebView (macOS/iOS): set drawsBackground on WebView, not config
• + "bdddb16" + — WebView (macOS/iOS): expose AppleWkWebView::withBackgroundColour

Expected consequences

• macOS/iOS: The transparent-WKWebView trick goes away. WKWebView renders against its default opaque background again. The white/grey flash on first reveal will likely return until we ship a different fix.
• Windows: The renderer's default background colour falls back to chromium's white default, and the host HWND class brush goes back to the JUCE default. Startup/hide-show flashes will likely return.
• API surface: + "AppleWkWebView::withBackgroundColour" + / + "getBackgroundColour" + are removed by the + "bdddb16" + revert. Any caller in + "minimal_core" + will fail to compile until updated — there's one call site in + "lib/managers/web_bridge/MKUEmbeddedWebView.cpp" + that needs to be dropped in a follow-up commit alongside the submodule bump.

Test plan

• Build Memory Rites on macOS with this branch pinned and confirm the webview is no longer pixelated on Retina displays / when dragging across monitors
• Build on Windows and confirm WebView2 startup still works (cosmetic flash expected)
• If pixelation goes away, design a non-regressing fix for the flash (likely a + "viewDidChangeBackingProperties" + override on the WKWebView subclass to keep + "layer.contentsScale" + in sync with the window's backing factor)

🤖 Generated with Claude Code

[jakepenn]

Reverting isn't the right path — adding a targeted viewDidChangeBackingProperties override on top of the existing patches instead. New PR incoming.