fix(filters): stop the channel list squashing its own rows flat

The picker's list is a flex column with a max height, so its fifty buttons were
flex children free to shrink: each collapsed to ~12px and clipped its own text
into an unreadable smudge instead of the box scrolling. shrink-0 on the rows.

Colour, contrast (9.03) and opacity all measured fine — the height was the
broken part, and no colour check was ever going to see that.

frontend/src/components/ChannelPicker.tsx

@@ -68,7 +68,9 @@ export default function ChannelPicker({
             <button
               key={c.id}
               onClick={() => setFilters({ ...filters, channelId: c.id, channelName: c.title ?? undefined })}
-              className="w-full text-left text-sm px-2 py-1.5 rounded-lg text-muted hover:text-fg hover:bg-card truncate transition"
+              // shrink-0 is load-bearing: these are flex children in a max-height box, so without it
+              // fifty of them squash to ~12px each and clip their own text instead of scrolling.
+              className="w-full shrink-0 text-left text-sm px-2 py-1.5 rounded-lg text-muted hover:text-fg hover:bg-card truncate transition"
             >
               {c.title ?? c.id}
             </button>