feat(theme): user-selectable custom fonts (#19)

Add a per-role font picker in Appearance settings. The two Material
typeface roles can be set independently: brand (display/headline) and
plain (title/body/label). Each can be the system default, a bundled
font (Atkinson Hyperlegible, Lora, JetBrains Mono), or a .ttf/.otf the
user loads from the device. The choice rebuilds the app-wide typography
in MainActivity; a null/"system" choice leaves the Material default.

Per the issue discussion the font-size and font-colour parts were
dropped — Android's accessibility font scaling and the dynamic-colour
theme already cover them.

Custom files are copied into app-private storage and validated with
android.graphics.fonts.Font.Builder before replacing the previous one,
so a bad pick can't wedge global text rendering; a missing/unreadable
file degrades to the system font. Each bundled font is previewed in its
own face in the picker. OFL licences + attribution vendored under
licenses/fonts/.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,52 @@
+package de.jeanlucmakiola.calendula.ui.theme
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.jupiter.api.Test
+
+class FontsTest {
+
+    @Test
+    fun `fromToken round-trips every bundled font`() {
+        BundledFont.entries.forEach { font ->
+            assertThat(BundledFont.fromToken(font.token)).isEqualTo(font)
+        }
+    }
+
+    @Test
+    fun `fromToken is null for system, custom, and unknown tokens`() {
+        assertThat(BundledFont.fromToken(FONT_SYSTEM_TOKEN)).isNull()
+        assertThat(BundledFont.fromToken(FONT_CUSTOM_TOKEN)).isNull()
+        assertThat(BundledFont.fromToken("not-a-font")).isNull()
+        assertThat(BundledFont.fromToken(null)).isNull()
+    }
+
+    @Test
+    fun `system for both roles returns the untouched default typography`() {
+        assertThat(calendulaTypography(brand = null, plain = null))
+            .isSameInstanceAs(CalendulaTypography)
+    }
+
+    @Test
+    fun `brand drives display and headline, plain drives title, body, and label`() {
+        val brand = BundledFont.Lora.family
+        val plain = BundledFont.JetBrainsMono.family
+        val typography = calendulaTypography(brand = brand, plain = plain)
+
+        assertThat(typography.displayLarge.fontFamily).isEqualTo(brand)
+        assertThat(typography.headlineSmall.fontFamily).isEqualTo(brand)
+        assertThat(typography.titleLarge.fontFamily).isEqualTo(plain)
+        assertThat(typography.bodyMedium.fontFamily).isEqualTo(plain)
+        assertThat(typography.labelSmall.fontFamily).isEqualTo(plain)
+    }
+
+    @Test
+    fun `a null role keeps that role's default family while the other is applied`() {
+        val plain = BundledFont.Lora.family
+        val typography = calendulaTypography(brand = null, plain = plain)
+
+        // Brand styles keep the Material default; plain styles switch.
+        assertThat(typography.displayLarge.fontFamily)
+            .isEqualTo(CalendulaTypography.displayLarge.fontFamily)
+        assertThat(typography.bodyLarge.fontFamily).isEqualTo(plain)
+    }
+}