feat(flow): filter the timeline by agent

Adds an agent filter to the FL0W header (#1473): a select listing the
live agents narrows the timeline to messages involving the chosen agent
(matched on `from` OR `to`). Each message row now carries
`data-from`/`data-to`; non-matching rows get `.flow-hidden`. New rows
pick up the active filter at render time; changing the filter re-scans
existing rows. The selection persists in localStorage so a reload or
tab-switch keeps the view. The dropdown is populated from the live
container list (and stays current on container add/remove); a saved
selection survives even if that agent isn't currently listed.

Pure frontend; composes with the sent+delivered collapse (the surviving
collapsed row keeps its `data-from`/`data-to`). Closes #1473.

frontend/packages/dashboard/src/flow.html

@@ -18,6 +18,9 @@
   <header class="logs-header">
     <a class="logs-back" href="/">← dashboard</a>
     <span class="logs-title">FL0W</span>
+    <select id="flow-agent-filter" class="flow-filter" title="filter timeline by agent" aria-label="filter timeline by agent">
+      <option value="">all agents</option>
+    </select>
   </header>
 
   <!-- Operator inbox flyout trigger — count + click → side panel