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]>

k5bss.html

@@ -30,7 +30,7 @@
     </div>
     <div class="header-status">
       <div class="status-item">
-        <span class="status-val" id="stat-qso">2,112</span>
+        <span class="status-val" id="stat-qso">—</span>
         <span class="status-lbl">QSOs Logged</span>
       </div>
       <div class="status-item">
@@ -59,6 +59,10 @@
 
 <main>
 
+  <div class="log-error" id="log-error" role="alert" hidden>
+    ⚠ LOG UNAVAILABLE — The logbook could not be loaded. Try again later.
+  </div>
+
   <!-- DASHBOARD -->
   <div id="section-dashboard" class="section active">
     <div class="dash-grid">