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/call.js

@@ -14,7 +14,7 @@
 // live-mutation paths call an injected `onCountsChanged` callback the entry
 // registers once via `initCall`.
 
-import { $, form, Panel, appendLinkified } from './common.js';
+import { $, form, appendLinkified } from './common.js';
 import { el } from '@hive/shared/dom.js';
 import { themedToast } from '@hive/shared/modal.js';
 import { epochSec, fmtAgo, fmtDuration } from './util.js';