fix(dashboard): constrain theme picker dropdown height so themes are scrollable (#25213) (#25220)

The header theme picker (`ThemeSwitcher`) renders a `role="listbox"` popup
with no `max-height` or overflow. With 20+ community themes installed under
`~/.hermes/dashboard-themes/`, the list extends past the viewport and themes
at the top or bottom are unreachable — the user reports only 15 of 26 themes
visible, with no scrollbar to access the rest.

Sibling switchers (`LanguageSwitcher`, `SlashPopover`) already cap their
listboxes (`max-h-80 overflow-y-auto` / `max-h-64 overflow-y-auto`); this
just brings the theme picker into line. Scoped to the component instead of
a global `div[role="listbox"]` CSS rule so other dropdowns aren't affected.

`70dvh` matches the user's tested workaround and the `dvh` unit handles
mobile browser UI chrome correctly (unlike `vh`).

Fixes #25213.

Co-authored-by: briandevans <252620095+briandevans@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/components/ThemeSwitcher.tsx

@@ -79,7 +79,7 @@ export function ThemeSwitcher({ dropUp = false }: ThemeSwitcherProps) {
           role="listbox"
           aria-label={t.theme?.title ?? "Theme"}
           className={cn(
-            "absolute z-50 min-w-[240px]",
+            "absolute z-50 min-w-[240px] max-h-[70dvh] overflow-y-auto",
             dropUp ? "left-0 bottom-full mb-1" : "right-0 top-full mt-1",
             "border border-current/20 bg-background-base/95 backdrop-blur-sm",
             "shadow-[0_12px_32px_-8px_rgba(0,0,0,0.6)]",