feat: add minimal web dashboard with stats, top credentials, and sessions

Implements Phase 1.5 — an embedded web UI using Go templates, Pico CSS
(dark theme), and htmx for auto-refreshing stats and active sessions.

Adds read query methods to the Store interface (GetDashboardStats,
GetTopUsernames, GetTopPasswords, GetTopIPs, GetRecentSessions) with
implementations for both SQLite and MemoryStore. Introduces the
internal/web package with server, handlers, templates, and tests.
Web server is opt-in via [web] config section and runs alongside
SSH with graceful shutdown. Bumps version to 0.2.0.

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

internal/web/templates/fragments/active_sessions.html

@@ -0,0 +1,26 @@
+{{define "active_sessions"}}
+<table>
+    <thead>
+        <tr>
+            <th>ID</th>
+            <th>IP</th>
+            <th>Username</th>
+            <th>Shell</th>
+            <th>Connected</th>
+        </tr>
+    </thead>
+    <tbody>
+        {{range .}}
+        <tr>
+            <td><code>{{truncateID .ID}}</code></td>
+            <td>{{.IP}}</td>
+            <td>{{.Username}}</td>
+            <td>{{.ShellName}}</td>
+            <td>{{formatTime .ConnectedAt}}</td>
+        </tr>
+        {{else}}
+        <tr><td colspan="5">No active sessions</td></tr>
+        {{end}}
+    </tbody>
+</table>
+{{end}}