fix(frontend): var-derive hardcoded palette colours for theme-swap safety

Replace ~92 hardcoded Catppuccin rgba literals across the dashboard,
agent, and shared terminal stylesheets with color-mix() over the theme
:root vars, so a stylix-generated theme.css recolours the whole UI, not
just the elements that already referenced the vars. Pixel-identical
under the default palette (same RGB + alpha; color-mix(in srgb, C N%,
transparent) == rgba(C, N/100)).

Also fixes four var(--mauve, #cba6f7) usages: --mauve is undefined (the
var is --purple), so they were silently falling back to the hardcoded
hex and would not have recoloured on a theme swap. The bare crust hex in
terminal.css and the checkerboard-gradient hexes in common.css are
var-derived too.

Pure black/white drop-shadow/frost scrims are left as literals (not
theme colours). Two off-palette surface tints (dim-purple schedule rows,
warm-amber inline-button hover) are also left, with a comment, pending a
dedicated named var. Correct-var fallbacks like var(--red, #f38ba8) are
left as-is since they already recolour via the var.

frontend/packages/dashboard/src/flow.css

@@ -10,7 +10,7 @@
      footer position. */
   --flow-header-h: 3.6em;
   --flow-composer-h: 3.6em;
-  --flow-frost-bg: rgba(30, 30, 46, 0.74);
+  --flow-frost-bg: color-mix(in srgb, var(--bg) 74%, transparent);
   --flow-frost-blur: blur(12px) saturate(140%);
 }
 
@@ -24,7 +24,7 @@ body.flow-shell {
   overflow: hidden;
   background:
     radial-gradient(ellipse 80% 60% at 50% 0%,
-                    rgba(203, 166, 247, 0.06) 0%,
+                    color-mix(in srgb, var(--purple) 6%, transparent) 0%,
                     transparent 60%),
     var(--bg);
 }