feat(webui): vue admin panel source

Vue 3 + TypeScript + Pinia + Tailwind v4 + shadcn-vue + Chart.js admin
panel source under webui/, built with Vite. Views: Overview (providers,
redis/cache/retry status, model aliases), Usage (per-user/per-group/total
tables with tokensIn/tokensOut/cost + limits), Charts (scope selector,
req/tokens(stacked in-out)/cost metric tabs, 24h/30d/12mo window
switcher, fed by GET /admin/api/usage/history). Pinia stores: auth
(sessionStorage key gate, x-api-key header, 401/403 re-prompt),
dashboard (5s poll), history (fetch-on-selection + 30s refresh).

Design: deliberate neutral warm-neutral palette with a signature teal
accent (no default Inter/purple-gradient/glassmorphism), real
typographic hierarchy via the platform UI/mono font stacks (zero
external assets - no Google Fonts), dark+light via prefers-color-scheme
(no class toggle, no theme switcher). Charts: zero-based bars, honest
axes, Okabe-Ito colorblind-safe series, tokens-in/out as a
distinguishable stacked pair.

vite.config.ts: base '/admin/' (matches admin.go's route layout) and
build.modulePreload.polyfill:false (no inline bootstrap script, so the
tightened admin CSP needs no 'unsafe-inline'). Code-split: Chart.js
(ChartsView) loads lazily, not in the initial bundle.

generate.mjs reads dist/ after `vite build` and writes
../admin_assets_gen.go (not committed by this change) - base64-encodes
every asset, decoded once at package-var init; documented rationale in
the script's own header comment.

Hygiene: webui/go.mod walls webui/ off from the repo root's `go build`/
`vet`/`test ./...` (npm's node_modules bundles a stray .go file that
would otherwise be swept in); tools/yaegi-check/main.go excludes webui/
from its GOPATH copy for the same reason. webui/ export-ignored
(.gitattributes) from release source tarballs; webui/node_modules/
gitignored. tsconfig.app.json/tsconfig.node.json's JSONC comments
dropped (TS itself is fine with either; the global pre-commit JSON
validator is not) - no config values changed.

webui/src/composables/useThemeColors.ts

@@ -0,0 +1,41 @@
+import { onBeforeUnmount, onMounted, ref } from 'vue'
+
+/**
+ * useThemeColors exposes the current computed values of the design
+ * tokens Chart.js needs to draw axis/grid/legend text (main.css) — Chart.js
+ * paints to a <canvas>, so it cannot pick up CSS variables the way a DOM
+ * element does; the caller must read them and hand them to Chart.js as
+ * plain color strings. Every color literal in main.css is oklch()/hex,
+ * both valid canvas fillStyle values in current evergreen browsers (the
+ * only ones this admin panel targets — see vite.config.ts's
+ * modulePreload note for the same reasoning), so no conversion is needed.
+ *
+ * Re-reads on a `prefers-color-scheme` change so an open Charts view
+ * repaints if the OS theme flips while it's on screen — main.css's own
+ * dark-mode block is a plain media query, not a class toggle, so this is
+ * the only way a chart already on screen learns about it.
+ */
+export function useThemeColors() {
+  const foreground = ref('')
+  const mutedForeground = ref('')
+  const border = ref('')
+
+  function sync(): void {
+    const style = getComputedStyle(document.documentElement)
+    foreground.value = style.getPropertyValue('--foreground').trim()
+    mutedForeground.value = style.getPropertyValue('--muted-foreground').trim()
+    border.value = style.getPropertyValue('--border').trim()
+  }
+
+  let media: MediaQueryList | undefined
+  onMounted(() => {
+    sync()
+    media = window.matchMedia('(prefers-color-scheme: dark)')
+    media.addEventListener('change', sync)
+  })
+  onBeforeUnmount(() => {
+    media?.removeEventListener('change', sync)
+  })
+
+  return { foreground, mutedForeground, border }
+}