feat: add server-side session filtering with input bytes and human score

Replace client-side session table filtering with server-side filtering
via a new /fragments/recent-sessions htmx endpoint. Add InputBytes column
to session tables, Human score > 0 checkbox filter, and Sort by Input
Bytes option to help identify sessions with actual shell interaction.

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

internal/web/templates/fragments/active_sessions.html

@@ -8,6 +8,7 @@
             <th>Username</th>
             <th>Type</th>
             <th>Score</th>
+            <th>Input</th>
             <th>Connected</th>
         </tr>
     </thead>
@@ -20,10 +21,11 @@
             <td>{{.Username}}</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>{{formatBytes .InputBytes}}</td>
             <td>{{formatTime .ConnectedAt}}</td>
         </tr>
         {{else}}
-        <tr><td colspan="7">No active sessions</td></tr>
+        <tr><td colspan="8">No active sessions</td></tr>
         {{end}}
     </tbody>
 </table>