fix(webui): keep charts consistent with the current selection

Chart and ranking fetches carry a request id, and a superseded response
is dropped. Changing a selection clears the old data at once, so old
numbers never show under new labels or units. The auto-refresh tick
skips while a fetch is still in flight. "Loading..." shows only until
the first result for a selection arrives. Dashboard polls do not
overlap.

Users in several groups are counted and searched in every group. The
Models tab labels its windows as the current UTC bucket. Hour labels
are marked UTC. The Members column sorts numerically. Target access
shows "All groups" for null and "No groups" for []. The UI shows the
server's own error message. Cost axis ticks use the currency
formatter. npm run dev proxies /admin/api. Nine unused shadcn
components are removed.

admin_assets_gen.go is regenerated from the new build.

Claude-Session: https://claude.ai/code/session_01V5jYJNxpDFdJkyyExpADrv

webui/src/assets/main.css

@@ -58,6 +58,14 @@
     --radius-md: calc(var(--radius) - 2px);
     --radius-lg: var(--radius);
     --radius-xl: calc(var(--radius) + 4px);
+    /* Powers the `font-heading` utility Tailwind v4 auto-generates from any
+       --font-* @theme token — CardTitle.vue and AlertTitle.vue apply it (a
+       review fix: they previously wrote the unrecognized class
+       `cn-font-heading`, matching nothing this token or Tailwind's engine
+       generates, so it rendered as a plain no-op string with zero styling
+       effect). Same font as body today; the token exists so a future
+       distinct heading face is a one-line change here, not a find-replace
+       across every component that uses it. */
     --font-heading: var(--font-sans);
 }