Colour markdown, which is the one language that is not tokens

The token scanner asks what a character is; markdown's meaning is where it
sits, so a `#` opens a heading at the start of a line and is an ordinary
character three words in. `MarkdownSyntax.kt` reads structure a line at a
time and then each line's prose left to right, and `spansOf` is the one
entry point that hides which of the two scanners a language got.

Conservative wherever a guess would be invisible: emphasis needs a closer on
the same line with no space beside either marker, so the `*p = *q` of a C
fragment opens nothing; an underscore may not start or end inside a word;
and an indented code block is left plain, since four spaces after a blank
line and four after a bullet are the same line.

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

app/androidApp/src/main/kotlin/com/example/aiapp/CodeFence.kt

@@ -201,6 +201,8 @@ private val FENCE_LANGUAGES: Map<String, Language> =
         "toml" to Language.TOML,
         "fish" to Language.FISH,
         "json" to Language.JSON,
+        "markdown" to Language.MARKDOWN,
+        "md" to Language.MARKDOWN,
     )
 
 /**