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/dashboard.html

@@ -66,6 +66,21 @@
                 </tbody>
             </table>
         </article>
+        <article>
+            <header>Top Exec Commands</header>
+            <table>
+                <thead>
+                    <tr><th>Command</th><th>Count</th></tr>
+                </thead>
+                <tbody>
+                    {{range .TopExecCommands}}
+                    <tr><td><code>{{truncateCommand .Value}}</code></td><td>{{.Count}}</td></tr>
+                    {{else}}
+                    <tr><td colspan="2">No data</td></tr>
+                    {{end}}
+                </tbody>
+            </table>
+        </article>
     </div>
 </section>
 
@@ -85,7 +100,7 @@
                 <th>IP</th>
                 <th>Country</th>
                 <th>Username</th>
-                <th>Shell</th>
+                <th>Type</th>
                 <th>Score</th>
                 <th>Connected</th>
                 <th>Disconnected</th>
@@ -94,11 +109,11 @@
         <tbody>
             {{range .RecentSessions}}
             <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>
                 <td>{{if .DisconnectedAt}}{{formatTime (derefTime .DisconnectedAt)}}{{else}}<mark>active</mark>{{end}}</td>