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/streams.html

@@ -1,6 +1,9 @@
 {{define "streams"}}{{template "header" .}}
 <div class="d-flex justify-content-between align-items-center mb-3">
-  <input type="text" class="form-control w-auto" placeholder="Search streams…" data-search-target="streamsTable">
+  <div class="input-icon">
+    <i class="bi bi-search"></i>
+    <input type="text" class="form-control" style="width:20rem" placeholder="Search streams…" data-search-target="streamsTable">
+  </div>
   <a href="/streams/new" class="btn btn-primary"><i class="bi bi-plus-lg"></i> Add Stream</a>
 </div>
 <div class="card">