fix(channels): stable tag column width, no popover flash, no sync wrap (R8 S1 UAT)

Three UAT refinements on the tag cell + popover platform:

- Fixed tag-column width (w-[15rem], was max-w): under table-layout:auto a content-
  driven cell re-solved every column's width on each chip added, so the table visibly
  shifted (and the "+N" count flickered) until the cap was hit. A fixed footprint makes
  adding/removing tags move nothing (verified: column x unchanged, "+N" count stable).
- No popover flash: gate the floating surface's visibility on Floating UI's isPositioned
  so it never paints at (0,0) top-left for a frame before the transform lands.
- Sync cell no longer wraps: flex-nowrap so the recent/full badges stay on one line
  (the column grows to fit them instead of breaking to two rows).

frontend/src/components/AnchoredPopover.tsx

@@ -64,7 +64,7 @@ export function useAnchoredPopover(opts: AnchoredPopoverOptions) {
   // null when not inside a <FloatingTree> — the hook still works, just without nested-layer awareness.
   const nodeId = useFloatingNodeId();
 
-  const { refs, floatingStyles, context, middlewareData } = useFloating({
+  const { refs, floatingStyles, context, middlewareData, isPositioned } = useFloating({
     nodeId,
     open: opts.open,
     onOpenChange: opts.onOpenChange,
@@ -108,6 +108,7 @@ export function useAnchoredPopover(opts: AnchoredPopoverOptions) {
     refs,
     floatingStyles,
     context,
+    isPositioned,
     referenceHidden: middlewareData.hide?.referenceHidden ?? false,
     ...interactions,
   };
@@ -134,8 +135,16 @@ export function PopoverSurface({
   focusManaged?: boolean;
   initialFocus?: number | React.MutableRefObject<HTMLElement | null>;
 }) {
-  const { context, refs, floatingStyles, getFloatingProps, nodeId, referenceHidden, open } =
-    popover;
+  const {
+    context,
+    refs,
+    floatingStyles,
+    getFloatingProps,
+    nodeId,
+    referenceHidden,
+    isPositioned,
+    open,
+  } = popover;
   const mergedRef = useMergeRefs([refs.setFloating, floatingRef ?? null]);
   const floatingProps = getFloatingProps();
 
@@ -149,8 +158,12 @@ export function PopoverSurface({
       style={{
         ...(floatingProps.style as CSSProperties | undefined),
         ...floatingStyles,
-        // Trigger scrolled out of its clipping scroller → hide-and-follow rather than float over chrome.
-        ...(referenceHidden ? { visibility: "hidden", pointerEvents: "none" } : null),
+        // Hide until (a) the first position is measured — else it flashes at top-left (0,0) before
+        // the transform lands — and (b) while the trigger is scrolled out of its clipping scroller
+        // (hide-and-follow rather than float over the chrome).
+        ...(!isPositioned || referenceHidden
+          ? { visibility: "hidden", pointerEvents: "none" }
+          : null),
         ...style,
       }}
     >