Let the frame corners be transparent; square them when maximized

The rounded frame only works if nothing paints an opaque full-bleed surface
behind it — the area outside a border-radius is not painted by the element,
so whatever sits underneath fills the corners. The frame comment asserted
that invariant ("nothing but .window paints an opaque surface") but nothing
enforced it: `html, body { background: var(--background) }` painted an
opaque rectangle across the whole window, so the corners had a ground to
show and the window read as a rectangle with a curve drawn on it.

Reset html/body to transparent under .win7. The override has to be
UNLAYERED, which is the non-obvious part: the html/body rule it overrides is
itself unlayered, and unlayered declarations beat every @layer regardless of
specificity, so the skinfix layer could not have reached it. Corrected the
frame comment, which stated the invariant as if it were free.

Maximized windows now drop the radius: a rounded maximized window leaves
four notches of desktop in the screen corners. title-bar.ts publishes its
existing `maximized` signal as `is-maximized` on its host, and the frame
reads it off its own direct child with :has(), avoiding a second
windowState subscription. The radius is declared three times in 7.css — on
.window and on both the ::before tint and the ::after glass layer — so all
three are zeroed; leaving one rounded would cut the corner off the square
frame.

NOT visually verified, and I could not verify it here: `import -window`
captures the window's own pixels without compositing, so a genuinely
transparent corner and an opaquely black one flatten to the same black in
the capture, and forcing png32 just fills alpha with 1. XWayland's root
window is not the composited desktop, and no compositor-level capture tool
(grim/scrot/maim) is installed. Both rules are confirmed present in the
built stylesheet; whether the corner now shows the desktop, and how the
maximized state looks, need eyes on the actual screen.

Co-Authored-By: Claude <noreply@anthropic.com>

ui/src/app/title-bar/title-bar.ts

@@ -28,6 +28,10 @@ import { RpcService } from '../rpc.service';
   selector: 'app-title-bar',
   host: {
     class: 'title-bar electrobun-webkit-app-region-drag',
+    // Published so the FRAME can react: a maximized Win7 window has square
+    // corners. The window root reads this off its own child via :has() rather
+    // than subscribing to windowState a second time — see styles.css.
+    '[class.is-maximized]': 'maximized()',
     '(dblclick)': 'toggle()',
   },
   template: `