Images sized themselves once at restyle time and never got
re-measured on a pure window/pane resize (no text change, no image
fingerprint change) - so they'd stay whatever width they were last
styled at. The engine already had the fix for exactly this shape of
problem for wide tables (a stamped .scrollableBlockFullRange
attribute triggers a targeted restyle on width change), and the
shared image-rendering helper already had a restyleOnWidthChange flag
to opt into it - just never passed at the image call sites. Wired it
on for both ![]() and ![[embed]] rendering.
Also makes the titlebar/toolbar strip blend into the sidebar's
background instead of reading as a separate bar, matching Mail/Notes/
Finder - titlebarAppearsTransparent + fullSizeContentView via a small
NSViewRepresentable, same "reach into NSWindow directly" pattern
applyMacAppearance() already uses since SwiftUI's WindowGroup has no
direct API for either.