feat: add session indicators and top exec commands to dashboard

Add visual indicators to session tables (replay badge when events exist,
exec badge for exec sessions) and a new "Top Exec Commands" table on the
dashboard. Includes EventCount field on Session, GetTopExecCommands on
Store interface, and truncateCommand template function.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/web/templates.go

@@ -50,6 +50,12 @@ func templateFuncMap() template.FuncMap {
 			}
 			return *s
 		},
+		"truncateCommand": func(s string) string {
+			if len(s) > 50 {
+				return s[:50] + "..."
+			}
+			return s
+		},
 	}
 }