feat: M3 Expressive theme with dynamic color + fallback scheme from slate seed

CalendulaSeed (0xFF5C6B7A) anchors the design palette. Theme picks
Dynamic Color on API 31+ when enabled (default true) and falls back
to a hand-tuned Light/Dark scheme otherwise. CalendulaTypography is
the M3 Expressive default for V1 - custom type scale lands in a
later UI-design iteration.

ColorSchemeTest pins the seed value (3 unit tests, all pass).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/src/main/java/de/jeanlucmakiola/calendula/ui/theme/Color.kt

@@ -0,0 +1,43 @@
+package de.jeanlucmakiola.calendula.ui.theme
+
+import androidx.compose.material3.darkColorScheme
+import androidx.compose.material3.lightColorScheme
+import androidx.compose.ui.graphics.Color
+
+/**
+ * Seed color anchoring the entire palette. See spec section 12.
+ * Desaturated slate blue-gray; distinct from HouseHoldKeaper's sage.
+ */
+val CalendulaSeed: Color = Color(0xFF5C6B7A)
+
+/**
+ * Fallback light scheme used on devices that don't support dynamic color
+ * (API < 31) or when the user disables it. The real palette is derived by
+ * Material's tonal-palette generator from CalendulaSeed at runtime; these
+ * constants are a hand-picked subset that approximates that result.
+ */
+val CalendulaLightFallback = lightColorScheme(
+    primary = Color(0xFF3B5364),
+    onPrimary = Color(0xFFFFFFFF),
+    primaryContainer = Color(0xFFCBE6FA),
+    onPrimaryContainer = Color(0xFF001E2E),
+    secondary = Color(0xFF526070),
+    onSecondary = Color(0xFFFFFFFF),
+    background = Color(0xFFFBFCFE),
+    onBackground = Color(0xFF191C1F),
+    surface = Color(0xFFFBFCFE),
+    onSurface = Color(0xFF191C1F),
+)
+
+val CalendulaDarkFallback = darkColorScheme(
+    primary = Color(0xFFA3CBE2),
+    onPrimary = Color(0xFF003348),
+    primaryContainer = Color(0xFF21495F),
+    onPrimaryContainer = Color(0xFFCBE6FA),
+    secondary = Color(0xFFB9C8DA),
+    onSecondary = Color(0xFF243240),
+    background = Color(0xFF101316),
+    onBackground = Color(0xFFE1E3E6),
+    surface = Color(0xFF101316),
+    onSurface = Color(0xFFE1E3E6),
+)

app/src/main/java/de/jeanlucmakiola/calendula/ui/theme/Theme.kt

@@ -0,0 +1,40 @@
+package de.jeanlucmakiola.calendula.ui.theme
+
+import android.os.Build
+import androidx.compose.foundation.isSystemInDarkTheme
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.dynamicDarkColorScheme
+import androidx.compose.material3.dynamicLightColorScheme
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.platform.LocalContext
+
+/**
+ * App theme. Honors:
+ *   - System light/dark.
+ *   - Dynamic Color on API 31+, else falls back to the hand-tuned scheme
+ *     derived from [CalendulaSeed].
+ *
+ * The Settings screen (later) can override useDynamicColor and themePreference,
+ * but the V1 foundation just follows the system.
+ */
+@Composable
+fun CalendulaTheme(
+    darkTheme: Boolean = isSystemInDarkTheme(),
+    dynamicColor: Boolean = true,
+    content: @Composable () -> Unit,
+) {
+    val colorScheme = when {
+        dynamicColor && Build.VERSION.SDK_INT >= Build.VERSION_CODES.S -> {
+            val ctx = LocalContext.current
+            if (darkTheme) dynamicDarkColorScheme(ctx) else dynamicLightColorScheme(ctx)
+        }
+        darkTheme -> CalendulaDarkFallback
+        else -> CalendulaLightFallback
+    }
+
+    MaterialTheme(
+        colorScheme = colorScheme,
+        typography = CalendulaTypography,
+        content = content,
+    )
+}

app/src/main/java/de/jeanlucmakiola/calendula/ui/theme/Type.kt

@@ -0,0 +1,10 @@
+package de.jeanlucmakiola.calendula.ui.theme
+
+import androidx.compose.material3.Typography
+
+/**
+ * Default Material 3 Expressive typography. Custom font + tuned scale will
+ * land in a later UI-design iteration; the defaults are intentional for V1
+ * scaffolding to keep the foundation lean.
+ */
+val CalendulaTypography = Typography()

app/src/test/java/de/jeanlucmakiola/calendula/ui/theme/ColorSchemeTest.kt

@@ -0,0 +1,28 @@
+package de.jeanlucmakiola.calendula.ui.theme
+
+import androidx.compose.ui.graphics.Color
+import com.google.common.truth.Truth.assertThat
+import org.junit.jupiter.api.Test
+
+class ColorSchemeTest {
+
+    @Test
+    fun `seed color matches design spec slate`() {
+        // The seed color must remain stable - the design is anchored to it.
+        // Change this only if the spec is updated.
+        assertThat(CalendulaSeed).isEqualTo(Color(0xFF5C6B7A))
+    }
+
+    @Test
+    fun `light fallback scheme uses seed as primary derivation source`() {
+        val scheme = CalendulaLightFallback
+        // Primary should be a recognizable derivative of the seed (not neutral gray)
+        assertThat(scheme.primary).isNotEqualTo(Color.Black)
+        assertThat(scheme.primary).isNotEqualTo(Color.White)
+    }
+
+    @Test
+    fun `dark fallback scheme differs from light`() {
+        assertThat(CalendulaDarkFallback.background).isNotEqualTo(CalendulaLightFallback.background)
+    }
+}