Glass panels catch the light at their edges (#51)

The glass surfaces had one 1px highlight along the top, which read as flat. They now have a rim
lit from the top-left and a fainter one on the far edges, as box-shadows, and a sheen from the
top-left corner as a background layer. A pseudo-element would have been the usual way, but the
detail pane, file list and dialogs scroll, and an absolutely placed layer in a scroller scrolls
away with the content.

Refraction was looked at and left out again: bending the live page needs backdrop-filter: url(),
which Firefox and Safari lack, and the WebGL libraries (liquid-glass-js, liquidGL) bend a
snapshot of the page that goes stale on a list that scrolls in its own pane.

The sheen lightens a dark panel under its text, so tests/contrast.js now checks every text colour
on a panel under the sheen at full strength. That capped it at 7% in dark mode; 9% put --faint,
--accent and --bad under AA.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -17,6 +17,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   elsewhere they copy the link. A link that looks like it carries your own access to a paid
   feed asks before it goes anywhere.
 
+### Changed
+
+- The Glass theme's panels catch the light: a bright rim along the top and left edges, a fainter
+  one along the far edges, and a soft sheen from the top-left corner.
+
 ### Fixed
 
 - Swiping to the next or previous item takes a longer swipe, a quarter of the screen, so a