fix(themes): unify input & dropdown focus borders across themes (#1052)

* chore(themes): drop the dead installs field from RegistryTheme

The store no longer reads install counts (the themes registry stopped
emitting them), so this optional field on RegistryTheme was unused.

* fix(themes): give dropdown border/shadow tokens a cascade default

--border-dropdown and --shadow-dropdown were set by only two themes and had
no base fallback, so dropdown/popover borders rendered without a themed value
in every other theme. Default them in the semantic cascade next to the other
menu tokens; themes that set them explicitly still override.

* fix(themes): unify input focus rings, drop the double border

Text inputs draw their own border + box-shadow focus ring, but the global
:focus-visible outline stacked a second ring outside it — a double border on
every field. Suppress the outline for text inputs centrally; the specificity
(0,1,1) beats the global ring but loses to the colour-blind-safe themes'
[data-theme] *:focus-visible (0,2,0), so those keep their stronger AAA ring on
every field by design — the header search now carries that ring on its cluster
there too. Align the few input classes that had a weaker or missing ring to the
shared border + 3px accent-dim standard.

* docs(changelog): add #1052 input/dropdown focus border fixes

src/styles/components/ui-scale-slider.css

@@ -165,8 +165,8 @@
 
 .folder-col-filter-input:focus {
   outline: none;
-  border-color: color-mix(in srgb, var(--accent) 70%, var(--border-subtle));
-  box-shadow: 0 0 0 2px color-mix(in srgb, var(--accent) 20%, transparent);
+  border-color: var(--accent);
+  box-shadow: 0 0 0 3px var(--accent-dim);
 }
 
 .folder-col.folder-col--compact .folder-col-filter {