feat: stream real-time progress for agent SDK install/upgrade/uninstall

Replace the spinner-only UX with live log output during agent SDK
operations, matching the existing OpenCode plugin install experience.

Backend: emit structured events (started/log/completed/failed) via
EventEmitter during npm install and binary download. npm commands now
run with piped stdio for line-by-line streaming; binary downloads
report chunked progress every 1 MB.

Frontend: subscribe to `app://agent-install` events through a new
`useAgentInstallStream` hook and render a theme-aware log terminal
below the preflight checks panel.

Also fixes the install log container in both agent settings and the
OpenCode plugins modal: auto-scroll no longer shifts the outer page,
and colours now follow the active light/dark theme.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/types.ts

@@ -936,6 +936,14 @@ export interface PluginInstallEvent {
   payload: string
 }
 
+export type AgentInstallEventKind = "started" | "log" | "completed" | "failed"
+
+export interface AgentInstallEvent {
+  task_id: string
+  kind: AgentInstallEventKind
+  payload: string
+}
+
 // ─── Chat Channels ───
 
 export type ChannelType = "lark" | "telegram" | "weixin"