theme: add --muted-on-dim, a --muted anchored to --purple-dim instead of --bg

--muted is color-mix()'d toward --base00/--bg — correct for dimmed text
on the page background, wrong for dimmed text sitting on the elevated
--purple-dim fill (badge/dropdown-item backgrounds). Badge's label/caret
used bare --muted there and mara measured it at ~1.3:1 contrast against
her theme's badge fill, essentially invisible.

--muted-on-dim mixes toward --purple-dim instead, same contrast-floor
technique as --muted itself, just anchored to the surface it's actually
used on. Applied to .ui-badge-label/.ui-badge-caret and to Dropdown's
active-item description (same bug: its row bg is --purple-dim too).

Ceiling note: even plain --fg only reaches ~3.4:1 against --purple-dim
in mara's theme, short of formal 4.5:1 AA — that's the theme's own
limit, not something the anchor choice can fix on its own. 90% gets
close to that ceiling (~3:1, more than double the old ~1.3:1) while
keeping a hint of the label/value visual distinction.

frontend/packages/shared/src/dropdown/Dropdown.css

@@ -44,5 +44,10 @@
 }
 .ui-dropdown-item-desc {
   font-size: 0.8em;
-  color: var(--muted);
+  /* `--muted-on-dim`, not bare `--muted`: an *active* item's row bg is
+     `--purple-dim` (above), the same low-contrast-on-elevated-fill case
+     Badge.css hit — see theme.css's `--muted-on-dim` comment. Inactive
+     rows are transparent (falls through to `--bg-elev`), where the two
+     vars read close enough that this doesn't cost anything there. */
+  color: var(--muted-on-dim);
 }