feat(panel): adaptive monitoring poll intervals (v1.x closure phase 1)

Replace fixed 5 s hx-trigger polling with data-poll markers and panel.js
scheduling: 5 s while active, 30 s when idle, none when tab is hidden.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/middleware.go

@@ -17,7 +17,7 @@ const usernameKey ctxKey = 0
 // everything except a GET request carrying HX-Request: the four monitoring
 // fragments (/status/fragment, /mail-queue/body, /system-log/body,
 // /deliveries/rows)
-// poll every 5s regardless of whether anyone is looking at the tab, so
+// poll on a timer regardless of whether anyone is looking at the tab, so
 // counting those as activity would make "N days idle" mean "N days since a
 // browser tab was last open" instead.
 func (s *Server) requireAuth(next http.Handler) http.Handler {