Restyle admin UI to dark theme, fix Docker /data volume permissions

- Rework the admin UI (topbar, sidebar, cards, tables, badges, stat
  tiles) into a dark theme with a light/dark toggle, matching the
  visual style requested for the project.
- Fix Dockerfile: chown /data to the distroless nonroot uid/gid before
  the VOLUME declaration so a freshly created named volume is seeded
  with writable permissions instead of root:root (was causing
  "unable to open database file" on first run).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QTXFrNP1imCW7jtfL5SZrw

internal/web/templates/dashboard.html

@@ -3,25 +3,38 @@
 <div class="row g-3 mb-4">
   <div class="col-sm-6 col-lg-3">
     <div class="card stat-card h-100"><div class="card-body">
-      <div class="text-secondary small">Active Proxy Hosts</div>
+      <div class="d-flex align-items-start justify-content-between">
+        <div class="stat-label">Active Proxy Hosts</div>
+        <i class="bi bi-globe2 stat-icon"></i>
+      </div>
       <div class="stat-value">{{$d.ActiveProxyHosts}}</div>
     </div></div>
   </div>
   <div class="col-sm-6 col-lg-3">
     <div class="card stat-card h-100"><div class="card-body">
-      <div class="text-secondary small">TCP / UDP Streams</div>
+      <div class="d-flex align-items-start justify-content-between">
+        <div class="stat-label">TCP / UDP Streams</div>
+        <i class="bi bi-arrow-left-right stat-icon"></i>
+      </div>
       <div class="stat-value">{{$d.ActiveTCPStreams}} / {{$d.ActiveUDPStreams}}</div>
     </div></div>
   </div>
   <div class="col-sm-6 col-lg-3">
     <div class="card stat-card h-100"><div class="card-body">
-      <div class="text-secondary small">Requests / sec (1h avg)</div>
+      <div class="d-flex align-items-start justify-content-between">
+        <div class="stat-label">Requests / sec</div>
+        <i class="bi bi-speedometer2 stat-icon"></i>
+      </div>
       <div class="stat-value">{{printf "%.2f" $d.RequestsPerSecond}}</div>
+      <div class="stat-sub">1h average</div>
     </div></div>
   </div>
   <div class="col-sm-6 col-lg-3">
     <div class="card stat-card h-100"><div class="card-body">
-      <div class="text-secondary small">Requests (24h)</div>
+      <div class="d-flex align-items-start justify-content-between">
+        <div class="stat-label">Requests (24h)</div>
+        <i class="bi bi-graph-up stat-icon"></i>
+      </div>
       <div class="stat-value">{{$d.TotalRequestsToday}}</div>
     </div></div>
   </div>