Remove RAW_DATA fallback and show a log-unavailable message

- Drop the embedded 20-contact RAW_DATA sample; the logbook now comes
  only from data/k5bss-log.json
- Show a banner and table message when the log fails to load, and
  display "—" for the header stats
- Skip building the dashboard when there is no data (it crashed on an
  empty log) and guard the activity chart redraw on resize
- Compute the QSO count from the log instead of hardcoding it

Co-Authored-By: Claude Opus 5.5 <[email protected]>

css/k5bss-style.css

@@ -193,6 +193,19 @@
     padding: 2rem;
   }
 
+  .log-error {
+    background: var(--surface);
+    border: 1px solid var(--red);
+    border-radius: 4px;
+    color: var(--red);
+    font-family: var(--font-mono);
+    font-size: 0.85rem;
+    letter-spacing: 0.1em;
+    padding: 1rem 1.5rem;
+    margin-bottom: 1.5rem;
+  }
+  .log-error[hidden] { display: none; }
+
   .section { display: none; }
   .section.active { display: block; }