feat(channels): fixed-width tag cell with "+N" overflow + inline create (R8 S1)

Redesign the channel tag cell (the popover pilot's first real consumer) to fix two
UAT-reported bugs and two UX gaps:

- Jumping editor (bug 1): the "+" edit trigger now sits in a STABLE slot at the end,
  so toggling a tag (which grows the chip row) no longer moves the anchor — the editor
  popover stays put (verified dx/dy = 0).
- Whole-table reflow (bug 2): the cell is width-bounded (max-w inside the cell, since
  table-layout:auto ignores the column width — the documented channel/About trap) and
  single-line, collapsing overflow into a hoverable "+N" pill. Rows no longer wrap and
  reflow (verified uniform row heights).
- The "+" is ALWAYS present (even with zero tags), and its menu can CREATE a tag inline
  (type + Enter → create + attach) — tagging no longer requires the top-of-page dialog.

New shared ChannelTagsControl (used by table + cards; the detail page is Part 2) over the
R8 popover platform: a `menu` editor popover + a `hover` reveal-all popover. Greedy rem-safe
fit via ResizeObserver. `api.createTag` now returns Promise<Tag>.

frontend/src/lib/api/tags.ts

@@ -12,7 +12,7 @@ export interface Tag {
 
 export const tagsApi = {
   tags: (): Promise<Tag[]> => req("/api/tags"),
-  createTag: (t: { name: string; color?: string; category?: string }) =>
+  createTag: (t: { name: string; color?: string; category?: string }): Promise<Tag> =>
     req("/api/tags", { method: "POST", body: JSON.stringify(t) }),
   updateTag: (id: number, patch: { name?: string; color?: string }) =>
     req(`/api/tags/${id}`, { method: "PATCH", body: JSON.stringify(patch) }),