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/home.html

@@ -31,7 +31,7 @@
 
       <a class="home-tile" href="/flow.html">
         <span class="home-tile-label">Flow</span>
-        <span class="home-tile-desc">all-agents chat + operator inbox</span>
+        <span class="home-tile-desc">live all-agents message firehose</span>
       </a>
 
       <a class="home-tile" href="/logs.html">