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/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)
+    }
+}