feat(market): square grid cells (2x1 min), preserve grid on edit, sparkline interval

- Grid cells are now square (cellH = cellW); column count is responsive with a
  2-column minimum, and a group's minimum size is 2x1 cells. New groups default
  to 2x1; drag/resize/auto-place all enforce the 2-wide minimum.
- Fix: editing a group no longer resets its grid placement/size. The editor's
  Save now starts from the existing entry and only overwrites the form fields,
  so grid_col/row/w/h (and anything else off-form) are preserved.
- Add a per-group sparkline interval (Minute/Hour/Day/Week/Month) next to the
  Sparkline toggle. The price history (~1 sample/min) is resampled by averaging
  each interval's worth of minute-samples into one point.

PortfolioEntry gains sparklineInterval and defaults gridW=2. Persisted in
settings.json. Full-node + Lite build clean; ctest 1/1; hygiene clean.

Note: the app only accumulates in-session minute samples, so Hour/Day/Month
show points only once enough data has been collected; true long-range history
(instant day/month charts) would need a CoinGecko market_chart fetch, which can
be added separately.

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

src/config/settings.h

@@ -90,11 +90,12 @@ public:
         bool showValue = true;          // show the converted/fiat value on the card
         bool show24h = false;           // show the 24h % change (live-market bases only)
         bool showSparkline = false;     // show a price-trend sparkline (live-market bases only)
-        // Dashboard grid placement on the Market portfolio. col/row in grid cells (-1 = auto-place),
-        // w/h in cells (span). Set when the user drags/resizes a card.
+        int sparklineInterval = 0;      // 0=minute 1=hour 2=day 3=week 4=month (resample of price history)
+        // Dashboard grid placement on the Market portfolio. col/row in square grid cells
+        // (-1 = auto-place), w/h in cells (span; minimum 2x1). Set when the user drags/resizes.
         int gridCol = -1;
         int gridRow = -1;
-        int gridW = 1;
+        int gridW = 2;
         int gridH = 1;
     };