fix: customer-based board visibility, chat widget UX, atomized Subdivide, static cache-busting

- Board visibility (§4.4): developers now see tasks for any customer with a
  consultant who pooled them, not only tasks whose consultantId is in their
  pool. Fixes a published task being invisible to its own author who is both
  consultant and developer. Adds TestBoardVisibilityIsCustomerBased.
- Chat widget: working ✕ close, distinct (non-ghost) header buttons, reliable
  conversation switching, ← Back, title ellipsis.
- Atomization: atomized/imported subtasks now expose a Subdivide button.
- Static assets: serve with ETag + Cache-Control: no-cache so JS/CSS fixes
  reach clients immediately instead of being masked by stale max-age caching.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/static/js/chat-widget.js

@@ -25,13 +25,13 @@ if (document.body.dataset.loggedIn === '1' && location.pathname !== '/messages')
   panel.hidden = true;
   panel.innerHTML = `
     <header>
-      <span style="display:flex;align-items:center;gap:8px">
-        <button class="btn small ghost" id="cw-back" hidden aria-label="Back to conversations">←</button>
+      <span style="display:flex;align-items:center;gap:8px;min-width:0">
+        <button class="btn small" id="cw-back" hidden aria-label="Back to conversations">← Back</button>
         <strong id="cw-title">Messages</strong>
       </span>
-      <span>
-        <a class="btn small ghost" id="cw-full" href="/messages" aria-label="Open full messages">⤢</a>
-        <button class="btn small ghost" id="cw-close" aria-label="Close">×</button>
+      <span style="display:flex;gap:6px">
+        <a class="btn small" id="cw-full" href="/messages" aria-label="Open full messages" title="Open full messages">⤢</a>
+        <button class="btn small" id="cw-close" aria-label="Close" title="Close">✕</button>
       </span>
     </header>
     <div class="cw-body" id="cw-body"></div>