fix(tokens): correct About z-index and accent/dark-mode token reuse

- About modal overlay used a raw z-index: 1200, below the semantic
  --z-modal (3000)/--z-toast (4000) layers, so token-based surfaces would
  cover it. Consume var(--z-modal) instead.
- .btn-primary and .search-button sit on the accent gradient but coloured
  their text with --color-danger-text (only correct by coincidence, #fff).
  Use the purpose-built --color-on-accent so retinting danger can't break
  them. The other --color-danger-text uses are white-on-red and stay.
- --color-item-hover-blue/--color-item-hover-sky were light-only literals
  that would read wrong in dark mode; wrap them in light-dark().

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013J5koaCrHDMvS7uwpawLBN

frontend/src/lib/components/AppShell.svelte

@@ -1028,7 +1028,7 @@
 	.about-overlay {
 		position: fixed;
 		inset: 0;
-		z-index: 1200;
+		z-index: var(--z-modal);
 		display: flex;
 		align-items: center;
 		justify-content: center;