Extract naturalCardBaseDp and scaledDensity helpers so tests bind to real code

resolvedCardHeight previously routed through a two-branch if(isWide)
hidden inside the composable extension, so the regression tests for
the global-slider reach bug only exercised the inner resolveCardHeight
(which was never broken). Lift the aspect -> natural base mapping
into a pure naturalCardBaseDp(AspectRatio) function with an exhaustive
when, and test that directly. SQUARE and FOUR_THREE rows piggyback on
the TALL global by design; the override branch still handles their
absolute heightDp values (Music presets at 100dp, etc.).

MainContent and DisplaySizePage's preview now both call a single
scaledDensity(base, percent) helper instead of inlining Density(...)
constructions. The double-fontScale-multiply regression has a real
guard now: tests assert scaledDensity(2.0, 1.0, 150) returns density
3.0 and fontScale 1.0 (not 1.5), and a third test confirms an
accessibility fontScale survives a UI scale change.

Pure refactor + test sharpening; no behaviour change.

app/src/main/java/com/github/damontecres/wholphin/MainContent.kt

@@ -19,7 +19,6 @@ import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.RectangleShape
 import androidx.compose.ui.platform.LocalDensity
-import androidx.compose.ui.unit.Density
 import androidx.compose.ui.unit.dp
 import androidx.lifecycle.Lifecycle
 import androidx.lifecycle.compose.LifecycleEventEffect
@@ -41,6 +40,7 @@ import com.github.damontecres.wholphin.ui.setup.SwitchServerContent
 import com.github.damontecres.wholphin.ui.setup.SwitchUserContent
 import com.github.damontecres.wholphin.ui.util.InterfaceCustomization
 import com.github.damontecres.wholphin.ui.util.LocalInterfaceCustomization
+import com.github.damontecres.wholphin.ui.util.scaledDensity
 
 @Composable
 fun MainContent(
@@ -63,17 +63,13 @@ fun MainContent(
         val interfaceCustomization =
             remember(appPreferences) { InterfaceCustomization(appPreferences) }
         val baseDensity = LocalDensity.current
-        val scaledDensity =
+        val uiScaleDensity =
             remember(baseDensity, interfaceCustomization.uiScalePercent) {
-                val factor = interfaceCustomization.uiScalePercent / 100f
-                Density(
-                    density = baseDensity.density * factor,
-                    fontScale = baseDensity.fontScale,
-                )
+                scaledDensity(baseDensity, interfaceCustomization.uiScalePercent)
             }
         CompositionLocalProvider(
             LocalInterfaceCustomization provides interfaceCustomization,
-            LocalDensity provides scaledDensity,
+            LocalDensity provides uiScaleDensity,
         ) {
             NavDisplay(
                 backStack = backStack,