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/dashboard/src/common.css

@@ -366,99 +366,24 @@ code {
 }
 
 /* ─── side panel ─────────────────────────────────────────────────
-   Long content (file previews, diffs, journald, applied config)
-   opens in a drawer that swipes in from the right — used by both
-   /flow.html and the dashboard. */
-.side-panel {
-  position: fixed;
-  inset: 0;
-  z-index: 200;
-  pointer-events: none;
-}
-.side-panel-backdrop {
-  position: absolute;
-  inset: 0;
-  background: color-mix(in srgb, var(--crust) 55%, transparent);
-  opacity: 0;
-  transition: opacity 200ms ease;
-}
-.side-panel-drawer {
-  position: absolute;
-  top: 0;
-  right: 0;
-  bottom: 0;
-  width: min(42em, 95vw);
-  background: var(--bg-elev);
-  border-left: 1px solid var(--purple-dim);
-  display: flex;
-  flex-direction: column;
-  transform: translateX(100%);
-  transition: transform 220ms ease;
-  overflow: hidden;
-}
-.side-panel.open { pointer-events: auto; }
-.side-panel.open .side-panel-backdrop { opacity: 1; }
-.side-panel.open .side-panel-drawer { transform: translateX(0); }
-.side-panel-resize {
-  position: absolute;
-  left: 0;
-  top: 0;
-  bottom: 0;
-  width: 6px;
-  cursor: ew-resize;
-  opacity: 0;
-  transition: opacity 120ms;
-  background: var(--purple-dim);
-}
-.side-panel-resize:hover,
-body.side-panel-resizing .side-panel-resize {
-  opacity: 1;
-}
-body.side-panel-resizing .side-panel-resize {
-  background: var(--purple);
-}
+   Long content (file previews, diffs, journald, applied config) opens in
+   a drawer that swipes in from the right — used by both /flow.html and
+   the dashboard. The drawer chrome itself (backdrop, drawer, resize
+   handle, header, title, close button) is now the shared
+   `<hive-side-panel>` element (@hive/shared/side-panel.js); what's left
+   here is the drag cursor-override escape hatch (has to be a global,
+   light-DOM rule — it reaches every element on the page during a drag,
+   not just the shared element's own shadow tree) and the styling for
+   the panel's own content types, which the shared element's shadow tree
+   can never reach (see @hive/shared/side-panel/hive-side-panel.css's
+   header comment). `.side-panel-body` below is the compatibility class
+   common.js's `Panel` wrapper puts directly on its `<hive-side-panel>`
+   instance so these rules keep reaching the slotted content by ordinary
+   light-DOM descendant matching. */
 body.side-panel-resizing {
   user-select: none;
 }
 body.side-panel-resizing * { cursor: ew-resize !important; }
-.side-panel-head {
-  display: flex;
-  align-items: center;
-  gap: 0.6em;
-  padding: 0.6em 1em;
-  border-bottom: 1px solid var(--border);
-  flex: none;
-}
-.side-panel-title {
-  flex: 1;
-  font-size: 0.88em;
-  color: var(--muted);
-  letter-spacing: 0.04em;
-  overflow: hidden;
-  text-overflow: ellipsis;
-  white-space: nowrap;
-}
-.side-panel-close {
-  display: flex;
-  align-items: center;
-  justify-content: center;
-  background: none;
-  border: 1px solid var(--border);
-  border-radius: 3px;
-  color: var(--muted);
-  font-size: 1em;
-  line-height: 1;
-  padding: 0.15em 0.4em;
-  cursor: pointer;
-  transition: border-color 0.15s ease, color 0.15s ease;
-}
-.side-panel-close:hover { border-color: var(--red); color: var(--red); }
-.side-panel-body {
-  flex: 1;
-  overflow-y: auto;
-  padding: 0.8em 1em;
-  font-size: 0.88em;
-}
 .side-panel-body .md { color: var(--fg); line-height: 1.5; }
 .side-panel-body .md > :first-child { margin-top: 0; }
 .side-panel-body .md > :last-child { margin-bottom: 0; }