fix(richtext): preserve blank lines between aligned paragraphs; share span merging

Code-review fixes: an all-empty paragraph group (a blank line isolated by an
alignment split) emitted <p></p>, which the parser collapses — it now emits one
<br> per line so blank lines round-trip. The identical span-merge helper that
existed in both the parser and RichTextEditing is now a single shared
mergeSameValueSpans() in RichText.kt, and the private applyBlock/applyLink drop
their never-used default font resolver.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/src/main/kotlin/org/libremail/richtext/RichText.kt

@@ -102,3 +102,20 @@ internal fun lineMarker(line: String): String? = when {
     line.startsWith(QUOTE_PREFIX) -> QUOTE_PREFIX
     else -> ORDERED_PREFIX.find(line)?.value
 }
+
+/**
+ * Merges spans with the exact same style value that touch or overlap, yielding the canonical
+ * maximal-run form. Shared by the HTML parser and the editing ops so the two can never drift.
+ */
+internal fun mergeSameValueSpans(spans: List<RichSpan>): List<RichSpan> {
+    val merged = ArrayList<RichSpan>()
+    for (span in spans.sortedWith(compareBy({ it.start }, { it.end }))) {
+        val i = merged.indexOfLast { it.style == span.style && span.start <= it.end }
+        if (i >= 0) {
+            merged[i] = merged[i].copy(end = maxOf(merged[i].end, span.end))
+        } else {
+            merged.add(span)
+        }
+    }
+    return merged
+}

app/src/main/kotlin/org/libremail/richtext/RichTextEditing.kt

@@ -28,7 +28,7 @@ object RichTextEditing {
         val updated = if (isFullyStyled(sameKind.filter { it.style == style }, start, end)) {
             cleared
         } else {
-            mergeSameValue(cleared + RichSpan(start, end, style))
+            mergeSameValueSpans(cleared + RichSpan(start, end, style))
         }
         return content.copy(spans = (otherKinds + updated).sortedBy { it.start })
     }
@@ -160,20 +160,6 @@ private fun subtractRange(spans: List<RichSpan>, start: Int, end: Int): List<Ric
     }
 }
 
-/** Merges spans with the exact same style value that touch or overlap. */
-private fun mergeSameValue(spans: List<RichSpan>): List<RichSpan> {
-    val merged = ArrayList<RichSpan>()
-    for (span in spans.sortedWith(compareBy({ it.start }, { it.end }))) {
-        val i = merged.indexOfLast { it.style == span.style && span.start <= it.end }
-        if (i >= 0) {
-            merged[i] = merged[i].copy(end = maxOf(merged[i].end, span.end))
-        } else {
-            merged.add(span)
-        }
-    }
-    return merged
-}
-
 // --- block marker helpers ---
 
 private val ORDERED = Regex("^\\d+\\. ")

app/src/main/kotlin/org/libremail/richtext/RichTextHtml.kt

@@ -122,12 +122,17 @@ private fun emitQuote(sb: StringBuilder, content: RichTextContent, lines: List<L
 private fun emitParagraph(sb: StringBuilder, content: RichTextContent, lines: List<Line>, from: Int): Int {
     val align = alignFor(content, lines[from])
     sb.append(openBlockTag("p", align))
+    val inner = StringBuilder()
     var i = from
     while (i < lines.size && lines[i].kind == Kind.PARAGRAPH && alignFor(content, lines[i]) == align) {
-        if (i > from) sb.append("<br>")
-        sb.append(renderInline(content, lines[i].contentStart, lines[i].contentEnd))
+        if (i > from) inner.append("<br>")
+        inner.append(renderInline(content, lines[i].contentStart, lines[i].contentEnd))
         i++
     }
+    // A group of only empty lines would otherwise vanish on parse (the closing tag's newline is a
+    // no-op at a line start), so it emits one <br> per line to keep the blank lines round-trippable.
+    val allEmpty = (from until i).all { lines[it].contentStart >= lines[it].contentEnd }
+    sb.append(if (allEmpty) "<br>".repeat(i - from) else inner)
     sb.append("</p>")
     return i
 }

app/src/main/kotlin/org/libremail/richtext/RichTextHtmlParser.kt

@@ -218,7 +218,7 @@ internal class RichTextHtmlParser(private val html: String) {
         val len = out.length
         return RichTextContent(
             text = out,
-            spans = mergedSpans(spans.mapNotNull { it.clampedTo(len) }),
+            spans = mergeSameValueSpans(spans.mapNotNull { it.clampedTo(len) }),
             links = links.mapNotNull { it.clampedTo(len) },
             alignments = mergedAlignments(alignments.mapNotNull { it.clampedTo(len) }),
             images = images.filter { it.end <= len },
@@ -242,20 +242,6 @@ private fun RichAlignment.clampedTo(len: Int): RichAlignment? {
     return if (start < e) copy(end = e) else null
 }
 
-/** Merges same-style spans that touch or overlap, yielding the canonical maximal-run form. */
-private fun mergedSpans(spans: List<RichSpan>): List<RichSpan> {
-    val merged = ArrayList<RichSpan>()
-    for (span in spans.sortedWith(compareBy({ it.start }, { it.end }))) {
-        val i = merged.indexOfLast { it.style == span.style && span.start <= it.end }
-        if (i >= 0) {
-            merged[i] = merged[i].copy(end = maxOf(merged[i].end, span.end))
-        } else {
-            merged.add(span)
-        }
-    }
-    return merged
-}
-
 /** Merges same-alignment runs on adjacent lines (ranges separated by exactly the newline). */
 private fun mergedAlignments(alignments: List<RichAlignment>): List<RichAlignment> {
     val merged = ArrayList<RichAlignment>()

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

@@ -300,7 +300,7 @@ private fun applyBlock(
     value: TextFieldValue,
     marker: BlockMarker,
     linkColor: Color,
-    resolveFont: (String) -> FontFamily? = { null },
+    resolveFont: (String) -> FontFamily?,
 ): TextFieldValue {
     val result = RichTextEditing.toggleBlock(
         value.annotatedString.toRichContent(),
@@ -318,7 +318,7 @@ private fun applyLink(
     value: TextFieldValue,
     url: String,
     linkColor: Color,
-    resolveFont: (String) -> FontFamily? = { null },
+    resolveFont: (String) -> FontFamily?,
 ): TextFieldValue {
     val updated = RichTextEditing.applyLink(
         value.annotatedString.toRichContent(),

app/src/test/kotlin/org/libremail/richtext/RichTextHtmlTest.kt

@@ -187,6 +187,20 @@ class RichTextHtmlTest {
         )
     }
 
+    @Test
+    fun `blank line between differently aligned paragraphs survives round-trip`() {
+        val content = RichTextContent(
+            text = "a\n\nb",
+            alignments = listOf(RichAlignment(0, 1, RichAlign.CENTER), RichAlignment(3, 4, RichAlign.CENTER)),
+        )
+        // The empty middle group must emit an explicit <br> or the blank line vanishes on parse.
+        assertEquals(
+            "<p style=\"text-align:center\">a</p><p><br></p><p style=\"text-align:center\">b</p>",
+            RichTextHtml.toHtml(content),
+        )
+        assertRoundTrips(content)
+    }
+
     @Test
     fun `parser accepts start and end alignment synonyms`() {
         val restored = RichTextHtml.fromHtml("<p style=\"text-align:start\">a</p><p style=\"text-align:end\">b</p>")