feat(market): compact chart w/ time axis; floating portfolio + redesigned group cards

Follow-up to the Market tab rework, per feedback:

- Chart is more compact: a modest responsive height (~22% of the tab, floored
  at 110px) instead of stretching to fill.
- Chart now has a time axis. The price series is one sample per ~60s market
  refresh (RefreshScheduler::kPrice), so the x-axis is labelled in approximate
  minutes-ago ("~45m … ~15m … now") instead of only "Now" — you can now read
  the interval/span of each plot point.
- Portfolio: removed the top-level card background — the summary (fiat hero +
  24h change + BTC, DRGX balance + Z/T, ratio bar) now floats directly under the
  MY DRGX header, and the "GROUPS" heading is gone.
- Group cards redesigned: a max width (280px, shrinks on narrow tabs) so they
  float rather than stretch; the group name is larger (subtitle font) top-left;
  the DRGX + fiat amounts moved to the card's top-right corner; and more inner
  padding around the content.

Removed the now-unused portfolio_groups i18n key. 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

@@ -1119,7 +1119,6 @@ void I18n::loadBuiltinEnglish()
         strings_["market_pct_shielded"] = "%.0f%% Shielded";
         strings_["market_portfolio"] = "MY DRGX";
         strings_["portfolio_all_funds"] = "All funds";
-        strings_["portfolio_groups"] = "GROUPS";
         strings_["portfolio_manage"] = "Manage\xE2\x80\xA6";
         strings_["portfolio_manage_title"] = "Manage portfolio";
         strings_["portfolio_add_entry"] = "Add entry";