Let a voice message without a waveform be seeked

A voice message that arrives without amplitude data — bridged messages and
those sent by clients that omit it — is unusable. The waveform is empty, so
its measured width is zero: the progress overlay never draws, the cursor is
pinned to the left edge, and the touch handler rejects every event because
no x coordinate can fall inside a zero-width range. The message plays, but
there is no way to see where it is or to move it.

Fall back to a flat run of zero amplitudes sized to the canvas when there
is no waveform. Width, progress, cursor and seeking then all work exactly
as they do for a real waveform, and the existing minimum amplitude renders
those zeros as the same flat track a genuinely silent recording already
draws, so no new drawing code is needed.

Nothing changes for a message that has a waveform, and the model still
carries the empty list, which is the honest value. Computing a real
waveform from the audio is a separate piece of work.

libraries/designsystem/src/main/kotlin/io/element/android/libraries/designsystem/components/media/WaveformPlaybackView.kt

@@ -200,11 +200,15 @@ internal fun WaveformPlaybackViewPreview() = ElementPreview {
     }
 }
 
-private fun ImmutableList<Float>.normalisedData(maxSamplesCount: Int): ImmutableList<Float> {
+internal fun ImmutableList<Float>.normalisedData(maxSamplesCount: Int): ImmutableList<Float> {
     if (maxSamplesCount <= 0) {
         return persistentListOf()
     }
 
+    if (isEmpty()) {
+        return List(maxSamplesCount) { 0f }.toImmutableList()
+    }
+
     // Filter the data to keep only the expected number of samples
     val result = if (this.size > maxSamplesCount) {
         (0..<maxSamplesCount)

libraries/designsystem/src/test/kotlin/io/element/android/libraries/designsystem/components/media/WaveformNormalisationTest.kt

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2025 Element Creations Ltd.
+ * Copyright 2025 New Vector Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.libraries.designsystem.components.media
+
+import com.google.common.truth.Truth.assertThat
+import kotlinx.collections.immutable.persistentListOf
+import org.junit.Test
+
+class WaveformNormalisationTest {
+    @Test
+    fun `an empty waveform is padded to the requested number of samples`() {
+        val result = persistentListOf<Float>().normalisedData(30)
+        assertThat(result).hasSize(30)
+        assertThat(result.toSet()).containsExactly(0f)
+    }
+
+    @Test
+    fun `an empty waveform stays empty when there is no room to display it`() {
+        assertThat(persistentListOf<Float>().normalisedData(0)).isEmpty()
+        assertThat(persistentListOf<Float>().normalisedData(-1)).isEmpty()
+    }
+
+    @Test
+    fun `a waveform shorter than the display count is kept as it is`() {
+        val waveform = persistentListOf(0.1f, 0.2f, 0.3f)
+        assertThat(waveform.normalisedData(30)).containsExactly(0.1f, 0.2f, 0.3f).inOrder()
+    }
+
+    @Test
+    fun `a waveform longer than the display count is downsampled`() {
+        val waveform = persistentListOf(0.1f, 0.2f, 0.3f, 0.4f, 0.5f, 0.6f)
+        assertThat(waveform.normalisedData(3)).hasSize(3)
+    }
+}