Move per-message rendering logic from frontend JS to backend Rust.
A new stream_enrich::enrich() function stamps display fields onto
LiveEvent::Stream payloads at SSE-emit time (both live tail and history
replay), so the frontend can consume pre-computed fields instead of
re-implementing the dispatch logic in JavaScript.
Phase 1: backend stamps _icon/_summary/_category; client falls back to
its own JS tables when absent. Zero user-visible change.
- system events: _category (drop/thinking_tok/note/details) + _summary
and optional _body (commands_changed expands to a slash-cmd list)
- assistant tool_use entries: _icon + _summary per tool; rich tools
(Write, Edit, send, ask, answer) get _category: 'rich'
- enrichment applied in web_ui/stream.rs at emit time; DB stores raw
events so no migration is needed when enrichment logic changes
- idempotent: existing _-prefixed fields are left unchanged
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