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/styles/base/variables.css

@@ -368,8 +368,8 @@
     --color-item-active: light-dark(#f8d2ae, #5a5047);
     --color-item-selected: light-dark(#fff8f6, #39281a);
     --color-item-hover-accent: light-dark(#fff0ec, #3d342c);
-    --color-item-hover-blue: #f0f8ff;
-    --color-item-hover-sky: #e0f2fe;
+    --color-item-hover-blue: light-dark(#f0f8ff, #18293f);
+    --color-item-hover-sky: light-dark(#e0f2fe, #103048);
 
     /* Sidebar — already dark-leaning in both modes; dark mode goes deeper. */
     --color-sidebar-bg-from: light-dark(#2a3042, #0f172a);