feat(compose): add strikethrough to the formatting toolbar

The rich-text engine already fully supported RichStyle.Strikethrough
(HTML serialization, parsing, and rendering); only the toolbar control
was missing. Adds an "S" FormatButton next to Bold/Italic/Underline,
wired the same way (onToggleStyle + isStyled toggle state), plus the
format_strikethrough string resource for its onClickLabel.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/kotlin/org/libremail/ui/compose/RichTextEditor.kt

@@ -190,6 +190,13 @@ private fun FormattingToolbar(
             underline = true,
             onClick = { onToggleStyle(RichStyle.Underline) },
         )
+        FormatButton(
+            label = "S",
+            description = stringResource(R.string.format_strikethrough),
+            active = RichTextEditing.isStyled(content, start, end, RichStyle.Strikethrough),
+            strikethrough = true,
+            onClick = { onToggleStyle(RichStyle.Strikethrough) },
+        )
         FormatButton(
             label = "•",
             description = stringResource(R.string.format_bullet_list),
@@ -226,6 +233,7 @@ private fun FormatButton(
     fontWeight: FontWeight? = null,
     fontStyle: FontStyle? = null,
     underline: Boolean = false,
+    strikethrough: Boolean = false,
 ) {
     val colors = MaterialTheme.colorScheme
     val background = if (active) colors.secondaryContainer else Color.Transparent
@@ -244,7 +252,11 @@ private fun FormatButton(
             style = LocalTextStyle.current.copy(
                 fontWeight = fontWeight,
                 fontStyle = fontStyle,
-                textDecoration = if (underline) TextDecoration.Underline else null,
+                textDecoration = when {
+                    underline -> TextDecoration.Underline
+                    strikethrough -> TextDecoration.LineThrough
+                    else -> null
+                },
             ),
         )
     }

app/src/main/res/values/strings.xml

@@ -97,6 +97,7 @@
     <string name="format_bold">Bold</string>
     <string name="format_italic">Italic</string>
     <string name="format_underline">Underline</string>
+    <string name="format_strikethrough">Strikethrough</string>
     <string name="format_bullet_list">Bulleted list</string>
     <string name="format_numbered_list">Numbered list</string>
     <string name="format_quote">Block quote</string>