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

src/shared/transactions.js

@@ -7,8 +7,8 @@ const { log } = require("./log");
 
 function formatTxValue(val) {
     const parts = val.split(".");
-    if (parts.length === 1) return val;
-    const dec = parts[1].slice(0, 6).replace(/0+$/, "") || "0";
+    if (parts.length === 1) return val + ".0000";
+    const dec = (parts[1] + "0000").slice(0, 4);
     return parts[0] + "." + dec;
 }