Add custom service log file support with L key

- Add ServiceLogConfig structure for per-host service log paths
- Implement L key handler for custom log file viewing via tmux popup
- Update dashboard config to support service_logs HashMap
- Add tail -f command execution over SSH for real-time log streaming
- Update status line to show L: Custom shortcut
- Document configuration format in CLAUDE.md

Each service can now have custom log file paths configured per host,
accessible via L key with same tmux popup interface as journalctl.

dashboard/src/config/mod.rs

@@ -9,6 +9,7 @@ pub struct DashboardConfig {
     pub hosts: HostsConfig,
     pub system: SystemConfig,
     pub ssh: SshConfig,
+    pub service_logs: std::collections::HashMap<String, Vec<ServiceLogConfig>>,
 }
 
 /// ZMQ consumer configuration
@@ -39,6 +40,13 @@ pub struct SshConfig {
     pub rebuild_alias: String,
 }
 
+/// Service log file configuration per host
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ServiceLogConfig {
+    pub service_name: String,
+    pub log_file_path: String,
+}
+
 impl DashboardConfig {
     pub fn load_from_file<P: AsRef<Path>>(path: P) -> Result<Self> {
         let path = path.as_ref();