feat: add session indicators and top exec commands to dashboard

Add visual indicators to session tables (replay badge when events exist,
exec badge for exec sessions) and a new "Top Exec Commands" table on the
dashboard. Includes EventCount field on Session, GetTopExecCommands on
Store interface, and truncateCommand template function.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/web/templates/fragments/active_sessions.html

@@ -6,7 +6,7 @@
             <th>IP</th>
             <th>Country</th>
             <th>Username</th>
-            <th>Shell</th>
+            <th>Type</th>
             <th>Score</th>
             <th>Connected</th>
         </tr>
@@ -14,11 +14,11 @@
     <tbody>
         {{range .}}
         <tr>
-            <td><a href="/sessions/{{.ID}}"><code>{{truncateID .ID}}</code></a></td>
+            <td><a href="/sessions/{{.ID}}"><code>{{truncateID .ID}}</code></a>{{if gt .EventCount 0}} <mark>replay</mark>{{end}}</td>
             <td>{{.IP}}</td>
             <td>{{.Country}}</td>
             <td>{{.Username}}</td>
-            <td>{{.ShellName}}</td>
+            <td>{{if .ExecCommand}}<mark>exec</mark>{{else}}{{.ShellName}}{{end}}</td>
             <td>{{if .HumanScore}}{{if gt (derefFloat .HumanScore) 0.6}}<mark>{{formatScore .HumanScore}}</mark>{{else}}{{formatScore .HumanScore}}{{end}}{{else}}-{{end}}</td>
             <td>{{formatTime .ConnectedAt}}</td>
         </tr>