frontend: drop the Panel forwarding object and the side-panel-body compat class

Per mara's review: '2 and maybe 1, but 3 also sounds reasonable on first
glance' (against 3 options I posted). Doing 2 and 1, leaving open()/
openNamed() as-is (option 3, tentative only).

Both dashboard/common.js and agent/app.js now export/use the
<hive-side-panel> element instance directly (sidePanel) instead of a
thin Panel = { open, openNamed, refresh, close } object that existed
purely to keep the old call-site shape unchanged. All 6 real call sites
updated to call the element's own methods directly.

The .side-panel-body class each wrapper stamped onto its own instance,
purely so common.css/agent.css's pre-existing content-styling selectors
kept matching, is gone too -- those selectors now use the element's own
tag name as the root (hive-side-panel .md, hive-side-panel .agent-inbox),
which already uniquely identifies the light-DOM instance without a
compatibility class. Verified via headless Chromium/CDP that the
tag-name selectors resolve correctly with no class needed.

Drive-by: removed an unrelated dead Panel import in call.js.

frontend/packages/dashboard/src/tabs.js

@@ -276,8 +276,8 @@ window.marked = marked;
   // NOTIF.bind() (the enable/mute/unmute toggle UI) moved to /settings.html
   // with the S3TT1NGS page. The dashboard keeps NOTIF.show() for approval/
   // question notifications — it reads the browser permission + localStorage
-  // mute flag the settings page sets, so no bind() is needed here. Panel
-  // (backed by <hive-side-panel>) wires its own internals in
+  // mute flag the settings page sets, so no bind() is needed here. The
+  // side panel (<hive-side-panel>) wires its own internals in
   // connectedCallback — no bind() step needed either.
 
   // ─── live updates: dashboard event stream ──────────────────────────────