feat(theme): add color and typography definitions for the app

android-app/app/src/main/java/de/harheimertc/ui/theme/Color.kt

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+package de.harheimertc.ui.theme
+
+import androidx.compose.ui.graphics.Color
+
+// Primary (Tailwind)
+val Primary50 = Color(0xFFFEF2F2)
+val Primary100 = Color(0xFFFEE2E2)
+val Primary200 = Color(0xFFFECACA)
+val Primary300 = Color(0xFFFCA5A5)
+val Primary400 = Color(0xFFF87171)
+val Primary500 = Color(0xFFEF4444)
+val Primary600 = Color(0xFFDC2626)
+val Primary700 = Color(0xFFB91C1C)
+val Primary800 = Color(0xFF991B1B)
+val Primary900 = Color(0xFF7F1D1D)
+
+// Accent (Tailwind gray-ish palette)
+val Accent50 = Color(0xFFFAFAFA)
+val Accent100 = Color(0xFFF4F4F5)
+val Accent200 = Color(0xFFE4E4E7)
+val Accent300 = Color(0xFFD4D4D8)
+val Accent400 = Color(0xFFA1A1AA)
+val Accent500 = Color(0xFF71717A)
+val Accent600 = Color(0xFF52525B)
+val Accent700 = Color(0xFF3F3F46)
+val Accent800 = Color(0xFF27272A)
+val Accent900 = Color(0xFF18181B)
+
+// Other semantic colors
+val AppBackground = Color(0xFFFFFFFF)
+val AppSurface = Accent50
+val AppOnPrimary = Color(0xFFFFFFFF)
+val AppError = Color(0xFFB00020)

android-app/app/src/main/java/de/harheimertc/ui/theme/Theme.kt

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+package de.harheimertc.ui.theme
+
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.lightColorScheme
+import androidx.compose.material3.darkColorScheme
+import androidx.compose.runtime.Composable
+
+private val LightColors = lightColorScheme(
+    primary = Primary500,
+    onPrimary = AppOnPrimary,
+    primaryContainer = Primary100,
+    secondary = Accent500,
+    background = AppBackground,
+    surface = AppSurface,
+    error = AppError,
+)
+
+private val DarkColors = darkColorScheme(
+    primary = Primary300,
+    onPrimary = AppOnPrimary,
+    primaryContainer = Primary700,
+    secondary = Accent300,
+    background = Accent900,
+    surface = Accent800,
+    error = AppError,
+)
+
+@Composable
+fun HarheimerTheme(
+    darkTheme: Boolean = false,
+    content: @Composable () -> Unit
+) {
+    val colors = if (darkTheme) DarkColors else LightColors
+
+    MaterialTheme(
+        colorScheme = colors,
+        typography = AppTypography,
+        shapes = MaterialTheme.shapes,
+        content = content
+    )
+}

android-app/app/src/main/java/de/harheimertc/ui/theme/Typography.kt

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+package de.harheimertc.ui.theme
+
+import androidx.compose.material3.Typography
+import androidx.compose.ui.text.font.FontFamily
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.sp
+
+// Placeholder typography mapping. Replace with bundled Inter / Montserrat fonts when added.
+val AppTypography = Typography(
+    displayLarge = androidx.compose.ui.text.TextStyle(
+        fontFamily = FontFamily.Default,
+        fontWeight = FontWeight.SemiBold,
+        fontSize = 30.sp
+    ),
+    titleLarge = androidx.compose.ui.text.TextStyle(
+        fontFamily = FontFamily.Default,
+        fontWeight = FontWeight.Medium,
+        fontSize = 20.sp
+    ),
+    bodyLarge = androidx.compose.ui.text.TextStyle(
+        fontFamily = FontFamily.SansSerif,
+        fontWeight = FontWeight.Normal,
+        fontSize = 16.sp
+    ),
+    bodyMedium = androidx.compose.ui.text.TextStyle(
+        fontFamily = FontFamily.SansSerif,
+        fontWeight = FontWeight.Normal,
+        fontSize = 14.sp
+    ),
+    labelSmall = androidx.compose.ui.text.TextStyle(
+        fontFamily = FontFamily.SansSerif,
+        fontWeight = FontWeight.Medium,
+        fontSize = 12.sp
+    )
+)