feat: add lightning fees widget to dashboard

[reneaaron]

Summary

Adds a Lightning fees widget to the home dashboard to highlight how cheap lightning payments are.

• New GET /api/transactions/stats endpoint aggregating settled outgoing payments into totalVolume, totalFeesPaid, and numPayments (self-payments excluded — they never traverse the network and would dilute the rate toward zero).
• FeeRateWidget renders a volume-weighted average fee rate (fees ÷ volume) plus a one-line supporting sentence. Hidden until there's payment volume, so it never shows a hollow 0% to brand-new users.
• Flat layout, no title icon (matches the other dashboard cards), <0.01% formatting for tiny rates.

Screenshots

Notes

• Scaling: the aggregate query's WHERE is index-assisted (state, type) so it won't full-table-scan, but amount_msat/fee_msat aren't indexed, so each matching row is fetched for the SUM. Fine for typical hubs (thousands of payments); a code comment documents adding a covering index transactions(type, state, self_payment, amount_msat, fee_msat) before this needs to scale to millions. This is already a better pattern than the existing GetForwards widget, which loads all rows into memory.
• Works on SQLite + Postgres (plain SUM/COUNT, no DB-specific percentile — chose a weighted rate over avg/median deliberately).

Test plan

• go test ./api/ -run TestGetTransactionStats — verifies the sums and that pending/incoming/self-payment rows are excluded, plus the empty case.
• go build ./..., frontend tsc + eslint + prettier clean.
• Manual: widget renders on Home once there are settled outgoing payments.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch lightning-fees-widget

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