feat(ui): add portfolio and analytics dashboard

[fwseyi]

Adds /portfolio — a one-glance view of inflow/outflow, per-asset totals, and top counterparties, derived entirely client-side from activityStore. No new network calls.

Changes

• src/lib/portfolio.ts — pure derivation functions: filterByWindow, calcAssetTotals, calcMonthlyFlow, calcTopCounterparties
• src/pages/Portfolio.tsx — time filter tabs (7d/30d/90d/All), assets table, inline SVG bar chart (no charting dep, dark/light adaptive), top-5 counterparties list, EmptyState fallback, wallet-not-connected state matching Activity page pattern
• src/App.tsx — /portfolio route
• src/components/Header.tsx — nav link added after /activity
• src/i18n/en.json, es.json — nav.portfolio translation keys

Performance

Benchmark included per acceptance criteria — 500 entries × 4 window switches:

• Total elapsed: 5.345ms (avg 1.336ms/window) — well under the 100ms threshold
• Single window switch (30d): 0.618ms
• Fixed calcMonthlyFlow to use a module-level Intl.DateTimeFormat instance instead of per-entry instantiation, cutting per-call cost from ~35ms to sub-millisecond

Tests

29 new unit tests in portfolio.test.ts covering all four derivation functions (empty input, boundary conditions, multi-token/multi-month aggregation, tiebreakers, missing-data fallbacks) plus 2 benchmark tests. 135/135 tests passing overall, no regressions.

Validation

pnpm test:unit → 10 test files, 135 passed
prettier --check → clean

Closes #148

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[truthixify]

Merged. Portfolio derives entirely from the existing activity store with no added network calls, exactly as you described in your application. Strong first contribution @fwseyi.