Fix tx amount display to 4 decimal places, add relative time to tx detail

- Transaction values now use exactly 4 decimal places (was 6),
  matching balance display everywhere else
- Transaction detail view shows "2026-02-25 15:04:23 (23 days ago)"
  instead of just the ISO date
- Added Display Consistency policy to README

README.md

@@ -115,6 +115,16 @@ invisible hit targets, no bare text that happens to have a click handler. If it
 does something when you click it, it must look like it does something when you
 click it.
 
+#### Display Consistency
+
+The same data must be formatted identically everywhere it appears. Token and ETH
+amounts are always displayed with exactly 4 decimal places (e.g. "1.0500 ETH",
+"17.1900 USDT") — in balance lists, transaction lists, transaction details, send
+confirmations, and any other context. Timestamps include both an ISO datetime
+and a humanized relative age wherever shown. If a formatting rule applies in one
+place, it applies in every place. Users should never see the same value rendered
+differently on two screens.
+
 #### Language & Labeling
 
 All user-facing text avoids crypto jargon wherever possible: