cleanup: extract events/chat viewer inline styles + drop dead var

The events-viewer and chat-viewer modals were each carrying ~5 inline
styled elements (modal sizing, status text, list container, filter
row + filter input) duplicated between the two modals. Edits to one
viewer left the other drifting visually.

Extracted to a shared .viewer-modal* family in styles.css:
- .viewer-modal sets the column flex layout
- .viewer-modal-events / .viewer-modal-chat set their own sizing
- .viewer-modal-title / .viewer-modal-status / .viewer-modal-list +
  inline + chat list variants for the data area
- .viewer-modal-filter-row + .viewer-modal-filter-input for the
  chat viewer's filter

Zero visual change; just stops the two viewers from drifting and
unblocks future polish (skeleton states inside the list, sticky
filter row, etc.) without an inline-edit-by-inline-edit grind.

Side: removed lastArchiveStatsScannedAt module variable in
renderer-stats.ts. It was assigned in refreshArchiveStats but never
read anywhere — leftover from an early plan to compare against a
previous timestamp before refreshing. The renderer-rendered "Last
scan" line reads stats.scannedAt directly. Dead, removed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/index.html

@@ -115,24 +115,24 @@
 
     <!-- Events Viewer Modal -->
     <div class="modal-overlay" id="eventsViewerModal" role="dialog" aria-modal="true" aria-labelledby="eventsViewerTitle">
-        <div class="modal" style="max-width: 700px; max-height: 80vh; display:flex; flex-direction:column;">
+        <div class="modal viewer-modal viewer-modal-events">
             <button class="modal-close modal-close-localizable" aria-label="Close" onclick="closeEventsViewer()">x</button>
-            <h2 id="eventsViewerTitle" style="margin-top:0;"></h2>
-            <div id="eventsViewerStatus" style="color:var(--text-secondary); font-size:12px; margin-bottom:8px;"></div>
-            <div id="eventsViewerList" style="flex:1; overflow-y:auto; background: var(--bg-main); border:1px solid var(--border-soft); border-radius:6px; padding:8px;"></div>
+            <h2 id="eventsViewerTitle" class="viewer-modal-title"></h2>
+            <div id="eventsViewerStatus" class="viewer-modal-status"></div>
+            <div id="eventsViewerList" class="viewer-modal-list"></div>
         </div>
     </div>
 
     <!-- Chat Replay Viewer Modal -->
     <div class="modal-overlay" id="chatViewerModal" role="dialog" aria-modal="true" aria-labelledby="chatViewerTitle">
-        <div class="modal" style="max-width: 800px; height: 80vh; display:flex; flex-direction:column;">
+        <div class="modal viewer-modal viewer-modal-chat">
             <button class="modal-close modal-close-localizable" aria-label="Close" onclick="closeChatViewer()">x</button>
-            <h2 id="chatViewerTitle" style="margin-top:0;"></h2>
-            <div class="form-row" style="margin-bottom:8px; gap:8px; flex-wrap:wrap; align-items:center;">
-                <input type="text" id="chatViewerFilter" placeholder="Filter..." oninput="onChatViewerFilterChange()" style="flex:1; min-width:160px; background: var(--bg-card); border:1px solid var(--border-soft); border-radius:6px; padding:6px 10px; color:var(--text); font-size:13px;">
-                <span id="chatViewerStatus" style="color:var(--text-secondary); font-size:12px;"></span>
+            <h2 id="chatViewerTitle" class="viewer-modal-title"></h2>
+            <div class="viewer-modal-filter-row">
+                <input type="text" id="chatViewerFilter" class="viewer-modal-filter-input" placeholder="Filter..." oninput="onChatViewerFilterChange()">
+                <span id="chatViewerStatus" class="viewer-modal-status viewer-modal-status-inline"></span>
             </div>
-            <div id="chatViewerList" style="flex:1; overflow-y:auto; background: var(--bg-main); border:1px solid var(--border-soft); border-radius:6px; padding:8px; font-family: 'Consolas', monospace; font-size: 12px;"></div>
+            <div id="chatViewerList" class="viewer-modal-list viewer-modal-list-chat"></div>
         </div>
     </div>