feat(frontend): add FIFO cost-basis tax report with CSV export (Closes #605)

[waterWang]

Summary

Adds a FIFO cost-basis tax report to the portfolio UI (issue #605):

• lib/portfolio.ts — new computeFIFOTaxReport() FIFO engine: received events open acquisition lots priced at historical CoinGecko prices; sent events dispose against the oldest lot first and contribute realized gains/losses; remaining lots are valued at the latest event price for unrealized gains. Missing historical prices flag the position as unpriced instead of producing wrong numbers.
• lib/portfolio.ts — fetchHistoricalPrices() (CoinGecko market_chart) and historicalPriceLookup() (back-fill ~5 days for weekend/holiday gaps, live-price fallback).
• components/TaxReport.tsx (new) — loads payment history + historical prices, renders a summary card (realized / unrealized / assets), a per-asset table (remaining qty, cost basis, unrealized, lots, priced flag), and a CSV export of the report.
• pages/portfolio.tsx — Portfolio ⇄ Tax Report tab switcher.
• i18n — portfolio.tabOverview / portfolio.tabTaxReport / taxReport.* keys added to all 7 locale files.
• Tests — 5 FIFO engine unit tests (single lot, realized gain on disposal, oldest-lot-first ordering, unpriced flag, order-independence + swap as paired events) and 3 TaxReport component tests (connect prompt, empty state, summary render).

Implementation notes

• FIFO ordering is applied internally, so the feed direction doesn't matter.
• Swap/trade operations are surfaced as paired sent(out) + received(in) events and are taxed naturally by the engine.
• CoinGecko pricing only covers XLM/USDC (existing COINGECKO_ID_MAP); other assets are flagged as unpriced rather than guessed.
• Non-XLM/USDC transaction values in CSV are reported in the raw event amounts; fiat columns are computed for priced assets.

Verification

• NODE_ENV=test npx jest __tests__/portfolio.test.ts __tests__/TaxReport.test.tsx __tests__/PortfolioOverview.test.tsx __tests__/PortfolioAllocation.test.tsx → 39/39 pass.
• tsc --noEmit — only pre-existing lib/sdk-instance.ts errors (TS1127/TS1160, introduced by PR#918 on main, unrelated to this change).
• eslint clean on all changed files (pre-existing require() in portfolio.test.ts mock untouched).

Closes #605

Signed-off-by: waterWang water.wang@users.noreply.github.com

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[Topmatrixmor2014]

FIX THESE CONFLICTS

[Topmatrixmor2014]

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