feat v1.24.0: general settings (global fill palette) + per-space LQI toggle

- General settings dialog: fill colors grouped by mode (light on/off, temp
  cold/ok/hot, lqi weak/strong), each with its own opacity; lqi fill lerps
  between the endpoints; stored in settings.fill_colors (defaults omitted);
  space-card uses the same palette
- per-space show_lqi toggle (badges + room tooltip line), inherits the card's
  show_signal when unset
- fillColorsOf/lerpColor/roomFillStyle helpers (+4 tests), backend schemas,
  smoke_general_settings; TESTING.md updated in the same commit

src/styles.ts

@@ -874,6 +874,11 @@ export const cardStyles = css`
     .rrow .ract:hover { color: var(--hp-txt); }
     .rrow .ract.del:hover { color: #ff7a5c; }
 
+    .gsrow .gsl {
+      min-width: 150px;
+      font-size: 12.5px;
+      color: var(--hp-muted);
+    }
     .dialogwrap {
       background: rgba(0, 0, 0, 0.45);
       display: flex;