feat(market): portfolio editor — taller icon grid + custom color picker

- Reorder the Appearance column so Color sits above the icon grid, and let the
  icon grid fill the remaining height of the left column (via
  GetContentRegionAvail().y). Fixes the large empty space that sat below the
  color row and gives the icon picker much more room.
- Add a custom-color circle after the preset swatches: it shows the current
  color when it isn't a preset (else a "+"), and clicking it opens a
  ColorPicker3 popup. The picked RGB packs straight into the group's color.

New i18n key portfolio_custom_color. Full-node + Lite build clean; ctest 1/1;
hygiene clean. Rendering change — needs a screenshot check.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/util/i18n.cpp

@@ -1123,6 +1123,7 @@ void I18n::loadBuiltinEnglish()
         strings_["portfolio_icon"] = "Icon";
         strings_["portfolio_color"] = "Color";
         strings_["portfolio_no_icon"] = "None";
+        strings_["portfolio_custom_color"] = "Custom color\xE2\x80\xA6";
         strings_["portfolio_appearance"] = "APPEARANCE";
         strings_["portfolio_addresses_hdr"] = "ADDRESSES";
         strings_["portfolio_search"] = "Search addresses\xE2\x80\xA6";