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/static/js/app.js

@@ -2,6 +2,24 @@
 (function () {
   "use strict";
 
+  // Light/dark theme toggle, persisted in localStorage.
+  var themeBtn = document.querySelector("[data-theme-toggle]");
+  if (themeBtn) {
+    var icon = themeBtn.querySelector("i");
+    function syncIcon() {
+      var isLight = document.documentElement.getAttribute("data-bs-theme") === "light";
+      icon.className = isLight ? "bi bi-sun" : "bi bi-moon-stars";
+    }
+    syncIcon();
+    themeBtn.addEventListener("click", function () {
+      var current = document.documentElement.getAttribute("data-bs-theme") === "light" ? "light" : "dark";
+      var next = current === "dark" ? "light" : "dark";
+      document.documentElement.setAttribute("data-bs-theme", next);
+      localStorage.setItem("vp-theme", next);
+      syncIcon();
+    });
+  }
+
   // Generic delete-confirmation modal. Any element with
   // data-confirm-delete-form="<form id>" and data-confirm-message="..."
   // opens #confirmDeleteModal and, on confirm, submits the named form.