test(compose): add RichTextEditor unit + ComposeScreen UI coverage (#176)

Closes the #36 test-coverage gap left after the rich-text editor shipped:

- RichTextEditorTest.kt (new JVM unit test, 20 cases): exercises the
  Compose-editor glue in RichTextEditor.kt that had no direct coverage -
  applyStyle/applyBlock/applyLink, the AnnotatedString.toRichContent() <->
  RichTextContent.toAnnotatedString() round trip across every span/link/
  alignment/image/baseStyle channel, and the isStyled/hasBlock predicate
  FormattingToolbar uses to light up its buttons. All plain TextFieldValue/
  AnnotatedString/Color types, so it runs on the JVM with no emulator.
  applyBlock/applyLink go from private to internal so the test can reach
  them directly, mirroring applyStyle's existing internal visibility.

- ComposeScreenTest.kt (androidTest, following this file's existing
  createAndroidComposeRule + fake-repository harness): one case taps the
  bullet-list toolbar button and asserts the sent message carries the
  <ul><li> HTML (block markers apply to the caret's line, so no fragile
  on-device range selection is needed); another asserts every toolbar
  button's click-action label matches its string resource, verifying the
  accessibility claim (the labels are onClickLabel, not contentDescription).

Headings remain deliberately out of scope per the ticket - no model/
toolbar changes here.

Closes #36

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -297,7 +297,7 @@ internal fun applyStyle(
     return TextFieldValue(updated.toAnnotatedString(linkColor, resolveFont), value.selection)
 }
 
-private fun applyBlock(
+internal fun applyBlock(
     value: TextFieldValue,
     marker: BlockMarker,
     linkColor: Color,
@@ -315,7 +315,7 @@ private fun applyBlock(
     )
 }
 
-private fun applyLink(
+internal fun applyLink(
     value: TextFieldValue,
     url: String,
     linkColor: Color,