Show an unreachable origin in the web UI (step 19)

A watcher that cannot fetch leaves the server perfectly reachable and
every branch row stale — a calm list that implies nothing changed. The
state was already recorded and served at /api/watcher; only nothing
rendered it.

The banner lives in the shared nav fragment, so every view inherits it,
and is fed from the view's own polling tick without being chained to it.
It stays separate from live-indicator: that one reports whether the
browser reaches the server, this one whether the server reaches origin.

Also caps the log volume the outage exposed: a lasting fetch failure is
logged when its message changes, not on every cycle, and the recovery is
logged once. Same for an invalid atTimes slot.

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

src/main/resources/templates/fragments.html

@@ -16,7 +16,8 @@
     <span class="repo-name" th:unless="${#strings.isEmpty(repoName)}" th:text="${repoName}">owner/repo</span>
 </h1>
 
-<div th:fragment="nav(view)" class="view-row">
+<th:block th:fragment="nav(view)">
+<div class="view-row">
     <nav class="view-toggle">
         <span th:if="${view == 'latest'}">Latest</span>
         <a th:unless="${view == 'latest'}" href="/">Latest</a>
@@ -33,6 +34,10 @@
         <button id="reload-button" class="reload-button" type="button" title="Reload view" aria-label="Reload view">⟳</button>
     </span>
 </div>
+<!-- Filled by gittally.js from /api/watcher: while the watcher cannot reach origin, every
+     row below is as old as its last successful poll, and a calm list would imply otherwise. -->
+<div id="watcher-banner" class="watcher-banner" role="status" hidden></div>
+</th:block>
 
 <footer th:fragment="footer" class="site-footer">
     <strong><a href="/releases" title="Release notes"><em th:text="'GitTally v' + ${version}">GitTally</em></a></strong>