fix(frontend): stop Alpha Zoo headline rendering the count twice

[warren618]

Summary

Fixes #286 — the Alpha Zoo page renders the alpha count twice in its headline (e.g. "456 456 pre-built quant alphas across 4 zoos").

Root cause

frontend/src/pages/AlphaZoo.tsx:218 rendered a standalone {total} and then the translated alphaZoo.prebuiltAlpha string — which already interpolates {{count}}. So the number printed twice. The same line also carried a hardcoded, now-stale 452 fallback.

// before
{total > 0 ? total : 452} {i18n.t("alphaZoo.prebuiltAlpha", { count: total > 0 ? total : 452 })}

Fix

• Render only the translated string, with the live (filter-aware) total as the count.
• Drop the hardcoded 452 fallback; while alphas load, show a count-less loading variant gated on the existing loading flag (avoids a "0 pre-built…" flash).
• New alphaZoo.prebuiltAlphaLoading key added to both en.json and zh-CN.json (locale key sets stay in parity).

// after
{loading
  ? i18n.t("alphaZoo.prebuiltAlphaLoading")
  : i18n.t("alphaZoo.prebuiltAlpha", { count: total })}

Test Plan

• tsc --noEmit — clean
• vitest run — 22 files, 209 tests passed
• Locale key parity verified (no en-only / zh-only keys)

Closes #286