feat: add ETFs tab to Markets page (SPY, QQQ, TLT, GLD, EEM, IWM, HYG…)

- New 'etfs' asset class in WATCHLIST: 18 key ETFs used in Pattern Lab presets
  (SPY, QQQ, IWM, TLT, IEF, HYG, GLD, EEM, FXI, EWG, EWJ, EWU, EWZ,
   XLF, SMH, KWEB, UUP, BIL)
- Added 'etfs' tab between Indices and Equities in Markets page
- Extended AssetClass union type to include 'etfs'

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/Markets.tsx

@@ -12,6 +12,7 @@ const ASSET_CLASSES: { key: AssetClass; label: string; emoji: string }[] = [
   { key: 'metals', label: 'Metals', emoji: '🥇' },
   { key: 'agriculture', label: 'Agriculture', emoji: '🌾' },
   { key: 'indices', label: 'Indices', emoji: '📊' },
+  { key: 'etfs', label: 'ETFs', emoji: '🗂' },
   { key: 'equities', label: 'Equities', emoji: '📈' },
   { key: 'forex', label: 'Forex', emoji: '💱' },
 ]