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/css/app.css

@@ -659,7 +659,11 @@ legend {
   background-image: var(--dither);
   background-color: var(--surface2);
 }
-.chat-panel header strong { font-family: var(--font-display); font-size: 1rem; }
+.chat-panel header strong {
+  font-family: var(--font-display); font-size: 1rem;
+  white-space: nowrap; overflow: hidden; text-overflow: ellipsis;
+}
+.chat-panel header .btn { background: var(--surface); }
 .chat-panel .cw-body { flex: 1; overflow: auto; padding: 6px 10px; }
 .chat-panel .cw-conv {
   display: flex; justify-content: space-between; align-items: center; gap: 8px;