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/stats.html

@@ -0,0 +1,20 @@
+{{define "stats"}}
+<div class="stats-grid">
+    <article class="stat-card">
+        <h2>{{.TotalAttempts}}</h2>
+        <p>Total Attempts</p>
+    </article>
+    <article class="stat-card">
+        <h2>{{.UniqueIPs}}</h2>
+        <p>Unique IPs</p>
+    </article>
+    <article class="stat-card">
+        <h2>{{.TotalSessions}}</h2>
+        <p>Total Sessions</p>
+    </article>
+    <article class="stat-card">
+        <h2>{{.ActiveSessions}}</h2>
+        <p>Active Sessions</p>
+    </article>
+</div>
+{{end}}