Implement real-time terminal popup for system rebuild operations

- Add terminal popup UI component with 80% screen coverage and terminal styling
- Extend ZMQ protocol with CommandOutputMessage for streaming output
- Implement real-time output streaming in agent system rebuild handler
- Add keyboard controls (ESC/Q to close, ↑↓ to scroll) for popup interaction
- Fix system panel Build display to show actual NixOS build instead of config hash
- Update service filters in README with wildcard patterns for better matching
- Add periodic progress updates during nixos-rebuild execution
- Integrate command output handling in dashboard main loop

shared/src/protocol.rs

@@ -9,6 +9,17 @@ pub struct MetricMessage {
     pub metrics: Vec<Metric>,
 }
 
+/// Command output streaming message
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct CommandOutputMessage {
+    pub hostname: String,
+    pub command_id: String,
+    pub command_type: String,
+    pub output_line: String,
+    pub is_complete: bool,
+    pub timestamp: u64,
+}
+
 impl MetricMessage {
     pub fn new(hostname: String, metrics: Vec<Metric>) -> Self {
         Self {
@@ -19,6 +30,19 @@ impl MetricMessage {
     }
 }
 
+impl CommandOutputMessage {
+    pub fn new(hostname: String, command_id: String, command_type: String, output_line: String, is_complete: bool) -> Self {
+        Self {
+            hostname,
+            command_id,
+            command_type,
+            output_line,
+            is_complete,
+            timestamp: chrono::Utc::now().timestamp() as u64,
+        }
+    }
+}
+
 /// Commands that can be sent from dashboard to agent
 #[derive(Debug, Serialize, Deserialize)]
 pub enum Command {
@@ -55,6 +79,7 @@ pub enum MessageType {
     Metrics,
     Command,
     CommandResponse,
+    CommandOutput,
     Heartbeat,
 }
 
@@ -80,6 +105,13 @@ impl MessageEnvelope {
         })
     }
 
+    pub fn command_output(message: CommandOutputMessage) -> Result<Self, crate::SharedError> {
+        Ok(Self {
+            message_type: MessageType::CommandOutput,
+            payload: serde_json::to_vec(&message)?,
+        })
+    }
+
     pub fn heartbeat() -> Result<Self, crate::SharedError> {
         Ok(Self {
             message_type: MessageType::Heartbeat,
@@ -113,4 +145,13 @@ impl MessageEnvelope {
             }),
         }
     }
+
+    pub fn decode_command_output(&self) -> Result<CommandOutputMessage, crate::SharedError> {
+        match self.message_type {
+            MessageType::CommandOutput => Ok(serde_json::from_slice(&self.payload)?),
+            _ => Err(crate::SharedError::Protocol {
+                message: "Expected command output message".to_string(),
+            }),
+        }
+    }
 }