frontend/agent: drop JS dispatch tables, read backend _icon/_summary/_category

Phase 2 of hyperhive#2196. The backend now pre-computes enrichment fields
on every SSE event (stream_enrich.rs); the frontend reads them directly
instead of running its own dispatch logic.

Removed (~330 lines of JS):
- fmtArgsGeneric / TOOL_ICONS / toolIcon / fmtRoom / fmtUser / fmtToolUse

renderStream changes:
- system events: dispatch on v._category (drop/thinking_tok/note/details)
  + v._summary / v._body instead of per-subtype if-chains; status tick
  still overrides label client-side when stateName === 'compacting' since
  elapsed time is a wall-clock value the backend cannot know at emit time
- tool_use: use c._category === 'rich' for rich-renderer routing,
  c._icon / c._summary for flat rows

renderRichToolUse: toolIcon(name) -> c._icon (from backend enrichment)

stream_enrich.rs: also stamp _category: 'drop' on top-level
type=result / type=rate_limit_event so the frontend can use a single
_category check instead of separate type-based early returns

hive-agent/src/web_ui/stream.rs

@@ -84,7 +84,7 @@ pub(super) async fn events_stream(
         // (`_icon`, `_summary`, `_category`) so the frontend doesn't need to
         // duplicate the dispatch logic. The DB stores raw events; enrichment
         // is applied here so both the live tail and the history endpoint
-        // deliver the same shape (see `events_history` below).
+        // deliver the same shape (see `events_history` above).
         if let crate::events::LiveEvent::Stream(ref mut v) = ev.event {
             crate::stream_enrich::enrich(v);
         }