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.

.gitattributes

@@ -24,3 +24,9 @@ testdata/ export-ignore
 .github/ export-ignore
 *_test.go export-ignore
 .gitattributes export-ignore
+# webui/ is the Vue admin panel's SOURCE (Vite/npm project) — CI and a
+# release build never need node: admin_assets_gen.go (repo root, `make
+# admin-ui`'s committed output) is the only artifact the plugin build
+# actually depends on, exactly like vendor/ above is safe to drop because
+# nothing at the repo root imports it directly.
+webui/ export-ignore