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/session_detail.html

@@ -7,6 +7,7 @@
             <table>
                 <tbody>
                     <tr><td><strong>IP</strong></td><td>{{.Session.IP}}</td></tr>
+                    <tr><td><strong>Country</strong></td><td>{{.Session.Country}}</td></tr>
                     <tr><td><strong>Username</strong></td><td>{{.Session.Username}}</td></tr>
                     <tr><td><strong>Shell</strong></td><td>{{.Session.ShellName}}</td></tr>
                     <tr><td><strong>Score</strong></td><td>{{formatScore .Session.HumanScore}}</td></tr>