fix(channels): keep the About popover on-screen at high browser zoom

At elevated zoom the CSS viewport shrinks; a long channel About wanted its fixed
max-h-112 (448px) height, so when floating-ui flipped it above the trigger its
top rendered above the viewport edge, clipped and unreachable (the inner
overflow-y-auto can't scroll past its own top). Enable the popover's fitHeight so
the `size` middleware caps the panel to the available space. Because the surface
splits into an outer positioned element and an inner styled/scroll wrapper, the
middleware now also publishes the available height as a --fit-max-h custom
property the inner scroller consumes via maxHeight: min(28rem, var(--fit-max-h)),
keeping the 28rem design cap on a tall viewport and shrinking to fit on a short one.

frontend/src/components/AboutCell.tsx

@@ -28,6 +28,10 @@ export default function AboutCell({ text, channelId }: { text: string | null; ch
     offset: 6,
     // Pop instantly like the old cell did; the close grace lets the cursor reach the card.
     hoverDelay: { open: 0, close: 160 },
+    // Cap the panel to the space between the trigger and the viewport edge (publishes --fit-max-h) so a
+    // long About never runs its top off-screen — at high browser zoom the short viewport made a fixed
+    // max-height overflow above the top edge, where the scroll container can't reach.
+    fitHeight: true,
   });
 
   const { data: ch } = useQuery({
@@ -51,7 +55,10 @@ export default function AboutCell({ text, channelId }: { text: string | null; ch
       </span>
       <PopoverSurface
         popover={pop}
-        className="glass w-[480px] max-h-112 overflow-y-auto px-3.5 py-3 rounded-lg animate-[fadeIn_0.12s_ease]"
+        className="glass w-[480px] overflow-y-auto px-3.5 py-3 rounded-lg animate-[fadeIn_0.12s_ease]"
+        // min(design cap, available space): keeps the 28rem cap on a tall viewport, shrinks to fit on a
+        // short one (high zoom). --fit-max-h is set by the fitHeight `size` middleware once positioned.
+        style={{ maxHeight: "min(28rem, var(--fit-max-h, 28rem))" }}
       >
         {/* Full About once the detail loads; until then the list's truncated description. */}
         {ch ? (

frontend/src/components/AnchoredPopover.tsx

@@ -131,7 +131,14 @@ export function useAnchoredPopover(opts: AnchoredPopoverOptions) {
             size({
               padding: 8,
               apply({ availableHeight, elements }) {
+                // Cap the positioned element, AND publish the available space as a custom property so
+                // the INNER styled/scroll wrapper (PopoverSurface renders `className` on a child, not on
+                // the floating element) can cap itself too — e.g. `maxHeight: min(28rem, var(--fit-max-h))`
+                // keeps a caller's design cap while still shrinking to fit at high zoom / short viewports.
+                // Capping only the outer element would leave the inner overflow-y-auto box at its fixed
+                // height, spilling its top off-screen (the whole point of fitHeight).
                 elements.floating.style.maxHeight = `${availableHeight}px`;
+                elements.floating.style.setProperty("--fit-max-h", `${availableHeight}px`);
               },
             }),
           ]