ui: recolor icons blue → purple for BitcoinPurple branding

SVG gradient stops updated directly; PNG files processed with a
+65° HSV hue rotation on blue-range pixels (185–245°, sat > 0.15),
preserving transparency, black, and white unchanged

electrum/gui/icons/electrum_darkblue.svg

@@ -69,11 +69,11 @@
     <linearGradient
        id="linearGradient3987">
       <stop
-         style="stop-color:#1382ef;stop-opacity:1;"
+         style="stop-color:#8b5cf6;stop-opacity:1;"
          offset="0"
          id="stop4032" />
       <stop
-         style="stop-color:#0056c0;stop-opacity:1;"
+         style="stop-color:#5b21b6;stop-opacity:1;"
          offset="1"
          id="stop3991" />
     </linearGradient>

electrum/gui/icons/electrum_lightblue.svg

@@ -63,11 +63,11 @@
     <linearGradient
        id="linearGradient3987">
       <stop
-         style="stop-color:#41b3ec;stop-opacity:1;"
+         style="stop-color:#c4b5fd;stop-opacity:1;"
          offset="0"
          id="stop4032" />
       <stop
-         style="stop-color:#0581c4;stop-opacity:1;"
+         style="stop-color:#7c3aed;stop-opacity:1;"
          offset="1"
          id="stop3991" />
     </linearGradient>