frontend: unify dashboard + agent side panel into shared hive-side-panel

The dashboard's Panel singleton and the per-agent UI's own inline Panel
IIFE each had their own near-identical implementation of the right-side
slide-in drawer used for file previews, diffs, logs, and inbox/todo
lists. Both are now thin wrappers around a new <hive-side-panel>
shadow-DOM custom element in @hive/shared, following the same house
pattern as <hive-menu>: the element owns and builds all its structural
chrome itself (backdrop, drawer, resize handle, header, title, close
button) in connectedCallback, and only the caller's opaque content node
is projected in via a default <slot> so each package's own
content-type-specific CSS keeps reaching it.

Public API is the union of both originals: open(title, content),
openNamed(name, title, content), refresh(name, title, content),
close(), and currentOwner(). Drag-to-resize + localStorage width
persistence (ported verbatim from the dashboard's original
implementation, the only one of the two that had it) is now available
to both consumers by default — a deliberate behavior widening for the
agent UI, which didn't have resize before. Along the way, fixed a
latent bug in the ported CSS: the resize handle was setting a
--side-panel-w custom property that no width rule ever consumed, so
dragging never actually resized the drawer even though it looked wired
up; the new shared stylesheet's width rule reads it properly.

Each package's own global stylesheet keeps its content-specific rules
(common.css's .side-panel-body .md, agent.css's .side-panel-body
.agent-inbox) exactly where they were — those can never be reached from
the shared element's shadow tree, same architectural floor as
<hive-menu>'s item-row styling. Each wrapper applies a plain
'side-panel-body' compatibility class to its own <hive-side-panel>
instance so those existing selectors keep matching by ordinary
light-DOM descendant matching, with the shared element itself having no
knowledge of what that class name means.

Panel.bind() is gone from both packages' public API — the shared
element wires its own listeners in connectedCallback, so there's no
bind step left to call. tabs.js's one call site (the only bind() caller
in either package) was updated to drop it.

The two original chrome CSS blocks disagreed on several purely visual
details beyond the resize-handle rules (z-index, backdrop color, drawer
border/box-shadow, title typography) — the dashboard's values (the more
feature-complete of the two) were kept as canonical, which is a small
visible style change for the agent UI's panel chrome (thinner border,
no box-shadow, no bold purple title). Flagged for visibility since nothing
in the original two implementations called this out explicitly.

Verified with a real headless-Chromium/CDP harness (bundled the actual
component + built page CSS, served statically, drove via raw CDP) for
both usage shapes: open/close, backdrop-click dismiss, Escape dismiss,
refresh() owner-matching (no-op on wrong owner, applies on matching
owner), and drag-to-resize (drawer width updates live during drag and
persists to localStorage on release).

frontend/packages/agent/src/app.js

@@ -6,6 +6,7 @@ import { create as termCreate, linkify as termLinkify } from '@hive/shared/termi
 import { asyncBtn, bindAsyncForms } from '@hive/shared/forms.js';
 import { themedConfirm } from '@hive/shared/modal.js';
 import { el } from '@hive/shared/dom.js';
+import '@hive/shared/side-panel.js'; // registers <hive-side-panel> — side-effect import
 import { marked } from 'marked';
 import DOMPurify from 'dompurify';
 
@@ -37,43 +38,30 @@ window.marked = marked;
   bindAsyncForms(() => refreshState());
 
   // ─── side panel (singleton drawer for inbox + loose-ends flyouts) ──────
-  // Shared shape with the dashboard's panel. Candidate for extraction
-  // into @hive/shared once both surfaces stabilise.
-  const Panel = (() => {
-    const root = $('side-panel');
-    const titleEl = $('side-panel-title');
-    const bodyEl = $('side-panel-body');
-    /** Owner key (e.g. 'inbox', 'loose-ends'). Refresh hooks check
-     *  against this so a live event only re-renders the panel when
-     *  the matching view is actually visible. null when closed. */
-    let owner = null;
-    function open(name, title, content) {
-      owner = name;
-      titleEl.textContent = title;
-      bodyEl.replaceChildren(...(content ? [content] : []));
-      root.classList.add('open');
-      root.setAttribute('aria-hidden', 'false');
+  // Thin wrapper around the shared `<hive-side-panel>` element (see
+  // @hive/shared/side-panel.js) — created and appended to <body> lazily
+  // on first use, same pattern `themedToast` uses for its toast-stack
+  // container. This UI's `open(name, title, content)` always takes an
+  // owner name, so it maps straight onto the shared element's
+  // `openNamed`; `side-panel-body` is a plain compatibility class this
+  // wrapper puts on its own instance so `agent.css`'s existing
+  // `.side-panel-body .agent-inbox …` rules keep reaching the slotted
+  // content by ordinary light-DOM descendant matching.
+  let panelEl = null;
+  function ensurePanel() {
+    if (!panelEl) {
+      panelEl = document.createElement('hive-side-panel');
+      panelEl.classList.add('side-panel-body');
+      document.body.append(panelEl);
     }
-    function close() {
-      owner = null;
-      root.classList.remove('open');
-      root.setAttribute('aria-hidden', 'true');
-    }
-    function refresh(name, title, content) {
-      if (owner !== name) return;
-      titleEl.textContent = title;
-      bodyEl.replaceChildren(...(content ? [content] : []));
-    }
-    function bind() {
-      $('side-panel-close').addEventListener('click', close);
-      $('side-panel-backdrop').addEventListener('click', close);
-      document.addEventListener('keydown', (e) => {
-        if (e.key === 'Escape' && root.classList.contains('open')) close();
-      });
-    }
-    return { open, close, refresh, bind, currentOwner: () => owner };
-  })();
-  Panel.bind();
+    return panelEl;
+  }
+  const Panel = {
+    open: (name, title, content) => ensurePanel().openNamed(name, title, content),
+    close: () => ensurePanel().close(),
+    refresh: (name, title, content) => ensurePanel().refresh(name, title, content),
+    currentOwner: () => ensurePanel().currentOwner(),
+  };
 
   // Wire the header pills to open the side panel. Pre-built (vs
   // re-building per-click) so the freshest snapshot already lives