fix(channels): center table, kill popover top-left flash, exact "+N" count (R8 S1 UAT)

Second UAT pass on the tag cell + popover platform:

- Popover top-left flash (real root cause): the popIn keyframe animates `transform`,
  which overrode Floating UI's positioning `transform: translate(x,y)` for the animation's
  duration, parking the popover at (0,0). Move the animation to an INNER wrapper so the
  positioned element's transform stays clean (verified: menu is at the correct spot from
  frame 0). The isPositioned visibility gate is kept as a belt-and-suspenders.
- Table shifted right / not centered: my previous fixed-15rem tags column + nowrap Sync
  pushed the table past the max-w-[96rem] wrapper so it overflowed. Trim to 12rem tags and
  shorten the long "full history queued/coming" sync labels to "queued"/"coming" so the
  table fits the wrapper (1594 <= 1628) and centers again.
- "+N" count wrong/missing while adding tags: replace the iterative shrink (which lagged a
  render and could clip the pill's digits) with a deterministic measure — a hidden measurer
  lays out all chips + a sample pill, and we compute the exact fit in one pass. The count is
  always correct now (verified across varied rows), and a bare "+" with no number is
  structurally impossible.

frontend/src/components/Channels.tsx

@@ -465,7 +465,7 @@ export default function Channels() {
       cardLabel: false,
       // Bounded width so the cell can't widen the column and reflow the whole table; the control
       // fills it and collapses overflow into "+N" (see ChannelTagsControl).
-      width: "15rem",
+      width: "12rem",
       filter: {
         kind: "multi",
         options: userTags.map((tg) => ({ value: String(tg.id), label: tg.name })),