feat(richtext): parameterized styles, alignment/image/base-style channels, HTML round-trip

RichStyle becomes a sealed interface (Bold/Italic/Underline/Strikethrough +
FontFamily/FontSize/FontColor/Highlight); RichTextContent gains alignments,
images, and baseStyle channels. The HTML serializer emits merged <span style>
runs, text-align on <p>/<li> (splitting merged paragraphs at alignment
boundaries), <img src="cid:…"> over the visible [image: name] token, and a
single outer <div style> for the base style. The parser is a faithful inverse
and additionally tolerates <del>/<strike>, px font sizes, #rgb colors, and
start/end alignment synonyms; unknown CSS is ignored without dropping text.

hasFormatting() covers every new channel so ComposeViewModel.normalizedHtml()
never silently drops serialized formatting. The editor carries parameterized
style identity via string annotations (libremail:style / libremail:image), maps
alignment onto ParagraphStyle ranges, holds baseStyle in separate field state,
and RichTextEditing.toggleStyle now replaces a different value of the same kind
while styleAt() answers "current value over the selection" for pickers.
ColorSwatchRow is added for the upcoming color/highlight dialogs. No UI change.

Closes #70

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

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

@@ -88,6 +88,7 @@
     <string name="format_link_url">Link address</string>
     <string name="format_link_apply">Link</string>
     <string name="format_link_needs_selection">Select some text first, then add a link.</string>
+    <string name="format_color_none">No color</string>
 
     <!-- Drafts -->
     <string name="drafts_title">Drafts</string>