feat: add GeoIP country lookup with embedded DB-IP Lite database (PLAN.md 4.3)

Embeds a DB-IP Lite country MMDB (~5MB) in the binary via go:embed,
keeping the single-binary deployment story clean. Country codes are
stored alongside login attempts and sessions, shown in the dashboard
(Top IPs, Top Countries card, Recent/Active Sessions, session detail).

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

internal/web/templates/fragments/active_sessions.html

@@ -4,6 +4,7 @@
         <tr>
             <th>ID</th>
             <th>IP</th>
+            <th>Country</th>
             <th>Username</th>
             <th>Shell</th>
             <th>Score</th>
@@ -15,13 +16,14 @@
         <tr>
             <td><a href="/sessions/{{.ID}}"><code>{{truncateID .ID}}</code></a></td>
             <td>{{.IP}}</td>
+            <td>{{.Country}}</td>
             <td>{{.Username}}</td>
             <td>{{.ShellName}}</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>
         {{else}}
-        <tr><td colspan="6">No active sessions</td></tr>
+        <tr><td colspan="7">No active sessions</td></tr>
         {{end}}
     </tbody>
 </table>