Desktop equivalent of #1650. The kernel WarningEvent (added in #1650
for high-severity warnings) already flows through eventize → desktop
sidecar → webview as a Tauri event, but desktop's protocol type and
App reducer didn't know about it — the event reached the renderer and
was silently dropped.

- Add WarningEvent to desktop/src/protocol.ts (matches the dashboard
  shape exactly: type, id, ts, turn, text, severity).
- ChatMessage union grows a "warning" kind; reducer adds a case that
  gates on severity === "high" (low-severity already dropped at
  eventize; defense-in-depth here in case future emit sites use the
  field differently).
- Thread renderer adds the warning branch — same .sys-event-row inline
  divider styling as dashboard (single dim line, 11px, opacity 0.7,
  flanked by hairline rules). Visually weak so several through a long
  session don't crowd the conversation.

Stacked on top of #1650 — desktop and dashboard now have feature parity
for compaction / abort / rate-limit / storm-stuck visibility without
the alarmist red error cards.

Follow-up not in this PR:
- Settings toggle "Show system events in thread" with config persistence,
  for users who want to fully hide them. Severity filter (high-only)
  already keeps the visible set to ~1-3 per session, so this is a
  preference, not a noise mitigation.

Follow-up to #1650. Completes the system-event surfacing work:

1. Desktop bridge — the kernel WarningEvent (added in #1650) already
   flowed through eventize → desktop sidecar → webview as a Tauri
   event, but desktop's protocol type and App reducer didn't know
   about it. Add WarningEvent to desktop/src/protocol.ts (matches
   dashboard shape), grow ChatMessage with a "warning" kind, reducer
   gated on severity === "high", thread renderer adds the
   .sys-event-row inline divider (same styling as dashboard).

2. Settings toggle — new config field thread.showSystemEvents
   (default true) with load/saveShowSystemEvents helpers in
   src/config.ts. Desktop sidecar pushes it in $settings and handles
   it in settings_save. Both SPAs grow Settings/SettingsEvent/
   SettingsPatch, reducer stores it, warning render gates on
   settings.showSystemEvents !== false. Settings panel adds a
   seg-ctrl row in the Behavior section, en + zh-CN strings.

Default remains "shown" — severity filter (high-only) already caps
visible events to ~1-3 per session, so this is a preference, not a
noise mitigation.

…moved, persisted usage stats, plan dispatch gate

Headline themes:
- Desktop: bundle the CLI-hosted React dashboard, retire Tauri+Preact duplicate (#1418)
- Config: drop preset abstraction; flash/pro are direct model selections (#1657, #1630)
- Stats: persist cumulative usage to session meta + auto-restore on startup (#1667, #1680, #1643, #1628)
- Plans: editMode="plan" enforced at the ToolRegistry dispatch gate (#1681); step advance fix (#1629)
- Context: fold once at turn start, drop pre-flight + byte-ceiling (#1642, #1646); collapsible compacted card (#1649)
- Subagents: per-skill flash/pro override + Settings UI (#1632)
- Desktop polish: sidebar drag-resize (#1688), responsive collapse (#1585), copy/edit overlay + msg-history nav (#1645), Esc closes modal not turn (#1685), QQ tab isolation (#1672), DiffCard for edits (#1662), theme-aware highlighting (#1655), system events toggle (#1654/#1650), macOS TCC inheritance (#1614), dashboard.enabled (#1612)
- Dashboard polish: persistent session URL (#1586, #1589, #1599), theme-aware highlighting (#1664), IME confirm-enter guard (#1689), code-fence lang fix (#1677), vendor chunk split (#1587), markdown table h-scroll (#1562)
- TUI: Alt+S input stash/recall; static history isolated from input rerenders (#1635); legacy mouse drop (#1637, #1648); multi-edit gated in review (#1647)
- Diff: SplitDiff column border holds under CJK (#1686)
- MCP: workspace roots passed to servers (#1625); codeCommand honors mcpServers (#1603)
- Config plumbing: (baseUrl, apiKey) resolved as a tuple (#1658); stale model id self-heal (#1663)

See CHANGELOG for the full list.