fix(channels): tag-cell review follow-ups — rename-aware fit, measured gap, close reveal-all

Address the /code-review findings on the tag cell:
- key the overflow-fit measurement on tag id+name so a RENAME (which changes chip
  width) re-runs the fit, not just add/remove;
- reserve the '+N' pill space using the MEASURED gap (gap-1 is rem, scales with the
  font slider) instead of a px constant;
- force the reveal-all hover popover's state closed when nothing is collapsed, so it
  can't linger open against an unmounted '+N' trigger.

frontend/src/components/ChannelTags.tsx

@@ -1,4 +1,4 @@
-import { useLayoutEffect, useRef, useState } from "react";
+import { useEffect, useLayoutEffect, useRef, useState } from "react";
 import { Check, Plus } from "lucide-react";
 import { useTranslation } from "react-i18next";
 import { type Tag } from "../lib/api";
@@ -146,7 +146,8 @@ export function ChannelTagsControl({
   const rowRef = useRef<HTMLDivElement | null>(null);
   const measureRef = useRef<HTMLDivElement | null>(null);
   const [visible, setVisible] = useState(attached.length);
-  const idsKey = attached.map((tg) => tg.id).join(",");
+  // Key on id AND name so a RENAME (which changes a chip's width) re-runs the fit, not just add/remove.
+  const tagsKey = attached.map((tg) => `${tg.id}:${tg.name}`).join("|");
 
   useLayoutEffect(() => {
     const row = rowRef.current;
@@ -158,7 +159,12 @@ export function ChannelTagsControl({
       if (!pill) return;
       const pillW = pill.offsetWidth;
       const chips = nodes.slice(0, -1);
-      const GAP = 4; // gap-1
+      // Measure the real rendered gap (gap-1 is rem — it scales with the font slider) off the box
+      // between the last chip and the sample pill, rather than assuming a px constant.
+      const lastChip = chips[chips.length - 1];
+      const gap = lastChip
+        ? Math.max(0, pill.offsetLeft - (lastChip.offsetLeft + lastChip.offsetWidth))
+        : 0;
       const avail = row.clientWidth;
       let count = chips.length;
       for (let i = 0; i < chips.length; i++) {
@@ -166,7 +172,7 @@ export function ChannelTagsControl({
         if (!chip) break;
         const right = chip.offsetLeft + chip.offsetWidth; // right edge within the measurer
         const moreAfter = i < chips.length - 1;
-        if (right > avail - (moreAfter ? pillW + GAP : 0)) {
+        if (right > avail - (moreAfter ? pillW + gap : 0)) {
           count = i;
           break;
         }
@@ -178,7 +184,7 @@ export function ChannelTagsControl({
     const ro = new ResizeObserver(compute);
     ro.observe(row);
     return () => ro.disconnect();
-  }, [idsKey]);
+  }, [tagsKey]);
 
   const hidden = attached.length - visible;
 
@@ -197,6 +203,11 @@ export function ChannelTagsControl({
     mode: "hover",
     placement: "bottom",
   });
+  // If tags change so nothing is collapsed anymore, the "+N" trigger unmounts — force the reveal-all
+  // popover's state closed so it can't linger open against a gone reference.
+  useEffect(() => {
+    if (hidden === 0 && overflowOpen) setOverflowOpen(false);
+  }, [hidden, overflowOpen]);
 
   const pillClass =
     "shrink-0 text-[10px] px-1.5 py-0.5 rounded-full bg-card text-muted border border-border";