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/dashboard.html

@@ -40,10 +40,25 @@
             <header>Top IPs</header>
             <table>
                 <thead>
-                    <tr><th>IP</th><th>Attempts</th></tr>
+                    <tr><th>IP</th><th>Country</th><th>Attempts</th></tr>
                 </thead>
                 <tbody>
                     {{range .TopIPs}}
+                    <tr><td>{{.Value}}</td><td>{{.Country}}</td><td>{{.Count}}</td></tr>
+                    {{else}}
+                    <tr><td colspan="3">No data</td></tr>
+                    {{end}}
+                </tbody>
+            </table>
+        </article>
+        <article>
+            <header>Top Countries</header>
+            <table>
+                <thead>
+                    <tr><th>Country</th><th>Attempts</th></tr>
+                </thead>
+                <tbody>
+                    {{range .TopCountries}}
                     <tr><td>{{.Value}}</td><td>{{.Count}}</td></tr>
                     {{else}}
                     <tr><td colspan="2">No data</td></tr>
@@ -68,6 +83,7 @@
             <tr>
                 <th>ID</th>
                 <th>IP</th>
+                <th>Country</th>
                 <th>Username</th>
                 <th>Shell</th>
                 <th>Score</th>
@@ -80,6 +96,7 @@
             <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>
@@ -87,7 +104,7 @@
                 <td>{{if .DisconnectedAt}}{{formatTime (derefTime .DisconnectedAt)}}{{else}}<mark>active</mark>{{end}}</td>
             </tr>
             {{else}}
-            <tr><td colspan="7">No sessions</td></tr>
+            <tr><td colspan="8">No sessions</td></tr>
             {{end}}
         </tbody>
     </table>