refactor(flow): drop operator-inbox UI, now lives on Y3R C4LL (#1483)

The operator inbox moved to the dashboard's Y3R C4LL tab in #1469
(◆ 1NB0X ◆ section, with per-message + mark-all read). FL0W goes back
to being the pure event firehose, so remove its now-redundant inbox UI:

- flow.js: drop the operatorInbox store, inboxAppendFromEvent,
  buildInboxListNode, renderInbox, the inbox-pill click wiring, and the
  onAnyEvent hook that fed them. The side panel was only used for the
  inbox flyout on this page, so drop Panel.bind() + the Panel import too.
- flow.html: remove the inbox pill, the offscreen inbox-section div, and
  the side-panel markup.
- flow.css: remove the .flow-pill* and .flow-inbox-headless rules.
- home.html: Flow tile desc → "live all-agents message firehose".
- docs/web-ui/dashboard.md: drop the FL0W 0PER4T0R 1NB0X section, point
  at Y3R C4LL, and fix the count-pill + /op-send descriptions.

The agent filter, sent→delivered collapse, compose box, and OS
notifications on operator-bound traffic are unaffected.

frontend/packages/dashboard/src/flow.html

@@ -23,16 +23,6 @@
     </select>
   </header>
 
-  <!-- Operator inbox flyout trigger — count + click → side panel
-       (singleton, declared below). Hidden until the inbox is non-
-       empty. Mirrors the agent page's header pill pattern. -->
-  <button type="button" id="inbox-pill" class="flow-pill" hidden
-          title="open operator inbox">
-    <span class="flow-pill-icon" aria-hidden="true">📬</span>
-    <span class="flow-pill-label">inbox</span>
-    <span class="flow-pill-count" id="inbox-pill-count">0</span>
-  </button>
-
   <!-- Main content: the full-viewport terminal. Padded for the
        overlay header + composer so the first/last rows stay
        reachable. -->
@@ -55,34 +45,7 @@
     </div>
   </footer>
 
-  <!-- Inbox rendered offscreen — kept in the DOM so flow.js's
-       renderInbox keeps working unchanged. The pill click handler
-       opens the side panel which displays a clone of the list. The
-       legacy section heading would otherwise be visible; hidden
-       here. -->
-  <div id="inbox-section" class="flow-inbox-headless" hidden>
-    <p class="meta">loading…</p>
-  </div>
-
-  <!-- Slide-in side panel. Singleton — JS swaps the title + body
-       and toggles `.open`. On this page only used to surface the
-       operator inbox flyout; the dashboard's other panel uses
-       (approval diffs, file previews, logs) don't apply here. -->
-  <div id="side-panel" class="side-panel" aria-hidden="true">
-    <div class="side-panel-backdrop" id="side-panel-backdrop"></div>
-    <aside class="side-panel-drawer" role="dialog" aria-modal="true"
-           aria-labelledby="side-panel-title">
-      <header class="side-panel-head">
-        <span class="side-panel-title" id="side-panel-title"></span>
-        <button type="button" class="side-panel-close" id="side-panel-close"
-                title="close (esc)">✕</button>
-      </header>
-      <div class="side-panel-body" id="side-panel-body"></div>
-    </aside>
-  </div>
-
-  <!-- Flow-specific bundle. Contains the broker terminal init, the
-       operator-inbox derived store, the inbox pill flyout, and the
+  <!-- Flow-specific bundle. Contains the broker terminal init + the
        @-mention composer. Tab renderers etc. live in
        `/static/tabs.js` which /flow.html doesn't load. -->
   <script type="module" src="/static/flow.js" defer></script>