refactor(RichTextViewer): move isMarkdown onto RichTextViewerState

Cleaner conceptual model: the parsed state object is the place where
all derived facts about the content live, so `isMarkdown` becomes a
field on `RichTextViewerState` (populated by `RichTextParser.parseText`
using the same cheap heuristic).

`RichTextViewer` now calls `CachedRichTextParser.parseText` once at the
top and dispatches on `state.isMarkdown` instead of running a separate
scan. `CachedRichTextParser.isMarkdown(content)` and its dedicated
`isMarkdownCache` go away — the single `richTextCache` carries the
decision alongside the parsed segments.

Inner callers (`DisplaySecretEmoji`, `MultiSetCompose` reaction
preview, `DisplayUncitedHashtags`) are unaffected: they continue to
receive a fully-parsed state with all segments populated.

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/richtext/RichTextParser.kt

@@ -224,6 +224,7 @@ class RichTextParser {
             customEmoji = emojiMap.toImmutableMap(),
             paragraphs = segments,
             tags = tags,
+            isMarkdown = isMarkdown(content),
         )
     }
 
@@ -411,6 +412,18 @@ class RichTextParser {
     }
 
     companion object {
+        // Cheap heuristic: stored on the parsed state so callers (e.g. RichTextViewer's
+        // markdown vs regular dispatch) can read the decision off the cached result instead
+        // of running a separate scan + maintaining a separate cache.
+        fun isMarkdown(content: String): Boolean =
+            content.startsWith("> ") ||
+                content.startsWith("# ") ||
+                content.contains("##") ||
+                content.contains("__") ||
+                content.contains("**") ||
+                content.contains("```") ||
+                content.contains("](")
+
         val longDatePattern: Regex = Regex("^\\d{4}-\\d{2}-\\d{2}$")
         val shortDatePattern: Regex = Regex("^\\d{2}-\\d{2}-\\d{2}$")
 

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/richtext/RichTextParserSegments.kt

@@ -33,6 +33,7 @@ class RichTextViewerState(
     val customEmoji: ImmutableMap<String, String>,
     val paragraphs: ImmutableList<ParagraphState>,
     val tags: ImmutableListOfLists<String>,
+    val isMarkdown: Boolean = false,
 )
 
 @Immutable