feat(channels): suggest your own tags for untagged channels from your tagging pattern (E8·S4)

Learn, per user tag, which auto TOPIC tags are characteristic of the channels
you've tagged it, then suggest that tag for still-untagged subscribed channels
whose topics cover enough of the profile. Language auto-tags are excluded from
the signal — near-universal within a language, they flooded every channel.

New GET /api/tags/suggestions (read-only, deterministic) backed by
app/sync/tagsuggest.py. Surfaced in the Channel Manager as dashed "ghost" chips
in the tags cell — one tap accepts (reuses the attach-tag endpoint), × dismisses
for the session — plus a header badge and a bulk "Accept N confident" (the
full-profile matches) behind a confirm. No silent writes (D4 = suggest-and-confirm).

Also reflow a pre-existing prettier drift in dateFilter.ts from E8·S2.

frontend/src/lib/queryKeys.ts

@@ -14,6 +14,9 @@ export const qk = {
   me: () => ["me"] as const,
   myStatus: () => ["my-status"] as const,
   tags: () => ["tags"] as const,
+  // Per-channel suggested user tags, learned from the manual-tag pattern (E8·S4). Separate root so
+  // accepting a suggestion can invalidate it without touching the whole tag catalog.
+  tagSuggestions: () => ["tag-suggestions"] as const,
 
   // --- feed / video ---
   // Every parameterized key guards on `!== undefined` (not truthiness), so "no arg → bare prefix