feat: web dashboard auto-start via SessionStart hook

- Default daemon feature: `cargo build --release` now includes serve command
- flowctl hook daemon-start: auto-starts daemon on SessionStart, PID management
- flowctl hook daemon-stop: auto-stops daemon on Stop/session end
- Dashboard at http://127.0.0.1:17319 (Leptos SSR)
- Duplicate start prevention via PID file check

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

Author: z23cc

flowctl/crates/flowctl-daemon/src/server.rs

@@ -126,7 +126,7 @@ mod tests {
     use tempfile::TempDir;
 
     fn test_setup() -> (TempDir, DaemonRuntime, flowctl_scheduler::EventBus) {
-        let tmp = TempDir::new().unwrap();
+        let _tmp = TempDir::new().unwrap();
         let flow_dir = tmp.path().join(".flow");
         let paths = DaemonPaths::new(&flow_dir);
         paths.ensure_state_dir().unwrap();