Truncate addresses by 2 chars to compensate for color dot width

Move truncateMiddle to helpers.js for reuse. Shorten displayed addresses
by 2 characters wherever a dot is shown: home view (40 char max), tx list
(maxAddr - 2), and address detail container (40ch width).

src/popup/views/helpers.js

@@ -99,6 +99,13 @@ function balanceLinesForAddress(addr) {
     return html;
 }
 
+function truncateMiddle(str, maxLen) {
+    if (str.length <= maxLen) return str;
+    if (maxLen < 5) return str.slice(0, maxLen);
+    const half = Math.floor((maxLen - 1) / 2);
+    return str.slice(0, half) + "\u2026" + str.slice(-(maxLen - 1 - half));
+}
+
 // 16 colors evenly spaced around the hue wheel (22.5° apart),
 // all at HSL saturation 70%, lightness 50% for uniform vibrancy.
 const ADDRESS_COLORS = [
@@ -139,4 +146,5 @@ module.exports = {
     balanceLinesForAddress,
     addressColor,
     addressDotHtml,
+    truncateMiddle,
 };