fix: use user's fiat currency and locale settings consistently

[BitHighlander]

Summary

• Replace all hardcoded $/USD/en-US formatting with the user's chosen currency and locale from FiatProvider
• Remove legacy formatUsd() function — all 10+ call sites now use useFiat().fmtCompact()
• Update AnimatedUsd component to derive symbol and separators from fiat context instead of hardcoding $ and ,
• Fix all 15 asset.json locale files: "$0.00 USD ({{count}})" → "{{zeroValue}} ({{count}})"
• Fix all 10 staking.json locale files: ~${{usd}} → ~{{fiatValue}}
• Add browser language auto-detection on first launch (navigator.language → supported language match)
• Backend reports (PDF/swap) now use stored number_locale and fiat_currency settings instead of hardcoded en-US
• ZcashPrivacyTab block numbers use user's locale for thousand separators
• ReportDialog dates and totals use user's locale

Files changed (38)

• Removed: formatUsd() from formatting.ts
• Updated: AnimatedUsd, SendForm, AssetPage, SwapDialog, BtcXpubSelector, EvmAddressSelector, StakingPanel, ZcashPrivacyTab, ReportDialog
• Backend: reports.ts, swap-report.ts
• i18n: 15 asset.json + 10 staking.json locale files
• Init: i18n/index.ts (browser locale detection)

Test plan

• Change currency in Settings → verify all amounts show correct symbol/format across Dashboard, AssetPage, SendForm, SwapDialog
• Change number format locale → verify thousand/decimal separators update in DonutChart, AnimatedUsd, token balances
• Set browser language to a supported non-English locale, clear localStorage, reload → verify auto-detection picks the language
• Generate a PDF report → verify currency formatting matches user settings
• Check Zcash privacy tab block numbers render with locale-appropriate separators
• Verify staking rewards display with non-USD currency selected