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-tauri/src/web/event_bridge.rs

@@ -50,6 +50,9 @@ pub enum EventEmitter {
     #[cfg(feature = "tauri-runtime")]
     Tauri(tauri::AppHandle),
     WebOnly(Arc<WebEventBroadcaster>),
+    /// Silent no-op emitter — drops all events. Used when streaming progress
+    /// is not needed (e.g. legacy non-streaming call paths).
+    Noop,
 }
 
 /// Unified event emission: sends to both Tauri webview and Web clients (if applicable).
@@ -70,5 +73,6 @@ pub fn emit_event(
         EventEmitter::WebOnly(broadcaster) => {
             broadcaster.send(event, &payload);
         }
+        EventEmitter::Noop => {}
     }
 }