agent: TodosPanel/InboxPanel flyouts + header pills

New Preact `SidePanel` drawer — deliberately not the shared
`<hive-side-panel>` shadow-DOM custom element, same rationale as
StatusChips' Badge/Dropdown. Drops that element's drag-to-resize +
localStorage width persistence for this first slice (fixed width via
CSS); reuses agent.css's existing `.agent-inbox`/`.inbox-*`/
`.side-panel-empty` content classes verbatim, only the drawer chrome
itself is new (SidePanel.css). Kept mounted regardless of open/closed
state (toggles the `open` prop) rather than conditionally rendered, so
the slide/fade CSS transitions actually fire on both open and close.

- InboxPanel: renders `state.inbox` (already polled by useAgentState)
  + a "mark all read" action — plain `fetch()` to the dashboard's
  `/api/agent/{name}/mark-all-read` (JSON response, same as app.js;
  unlike pause/resume this one doesn't need a form-submit workaround).
- TodosPanel: new `useTodos` hook (same 4s poll cadence as app.js's
  `refreshTodos`) + bulk select/mark-done via `api/todos/mark-done`.
- Both destructive-but-recoverable actions use a new `useConfirmClick`
  two-click-arm hook instead of the old shadow-DOM `themedConfirm`
  modal — lighter weight, no backdrop/dialog machinery needed for a
  "did you mean to click that" nudge.
- HeaderPill: the inbox/todos count triggers in the header's right
  cluster; Header.tsx gained a `pills` slot for them (mirrors the old
  markup's `.agent-header-pills` third column).
- types.ts: added `InboxRow`/`TodoRow`, mirroring
  `hive_sh4re::inbox::{InboxRow, LooseEnd}` (todo variant only —
  `/api/todos` never returns the others).

Screenshot-verified against a mock server exercising both panels.

frontend/packages/agent/src/components/HeaderPill.tsx

@@ -0,0 +1,26 @@
+// <HeaderPill> — the inbox/todos flyout triggers in the header's right
+// cluster. Reuses agent.css's existing `.hive-pill.header-pill…` rules
+// (loaded globally) — same "layout is a like-for-like port, the
+// component model is the change" approach as Header.tsx. Hidden
+// (renders nothing) at count 0, matching the old page's `pill.hidden =
+// count === 0`.
+export interface HeaderPillProps {
+  kind: 'inbox' | 'todos';
+  icon: string;
+  label: string;
+  count: number;
+  onClick: () => void;
+}
+
+export function HeaderPill({ kind, icon, label, count, onClick }: HeaderPillProps) {
+  if (count === 0) return null;
+  return (
+    <button type="button" class={`hive-pill header-pill header-pill-${kind}`} onClick={onClick} title={`open ${label} flyout`}>
+      <span class="header-pill-icon" aria-hidden="true">
+        {icon}
+      </span>
+      <span class="header-pill-label">{label}</span>
+      <span class="header-pill-count">{count}</span>
+    </button>
+  );
+}