Implement idle/active mode for scalable agent connections

- Add StartStream/StopStream/AgentStatus messages to protobuf
- Agent now starts in idle mode (heartbeat only, no capture)
- Agent enters streaming mode when viewer connects (StartStream)
- Agent returns to idle when all viewers disconnect (StopStream)
- Server tracks viewer IDs and sends start/stop commands
- Heartbeat mechanism with 90 second timeout detection
- Session API now includes streaming status and agent info

This allows 2000+ agents to connect with minimal bandwidth.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/src/api/mod.rs

@@ -17,6 +17,12 @@ pub struct SessionInfo {
     pub agent_name: String,
     pub started_at: String,
     pub viewer_count: usize,
+    pub is_streaming: bool,
+    pub last_heartbeat: String,
+    pub os_version: Option<String>,
+    pub is_elevated: bool,
+    pub uptime_secs: i64,
+    pub display_count: i32,
 }
 
 impl From<crate::session::Session> for SessionInfo {
@@ -27,6 +33,12 @@ impl From<crate::session::Session> for SessionInfo {
             agent_name: s.agent_name,
             started_at: s.started_at.to_rfc3339(),
             viewer_count: s.viewer_count,
+            is_streaming: s.is_streaming,
+            last_heartbeat: s.last_heartbeat.to_rfc3339(),
+            os_version: s.os_version,
+            is_elevated: s.is_elevated,
+            uptime_secs: s.uptime_secs,
+            display_count: s.display_count,
         }
     }
 }