feat(compose): auto-detect RTL text direction while writing

[shukiv]

What

The compose surface now auto-detects text direction: typing Hebrew, Arabic,
or any RTL script flips that paragraph to right-to-left, per paragraph, while
LTR text stays left-to-right. Mixed messages (some RTL lines, some LTR) each
align correctly.

How

Pure dir="auto" — the browser's built-in bidi detection picks direction from
each block's first strong character. No new dependency.

• rich-text-editor: the Paragraph and Heading nodes are extended with a
  dir attribute defaulting to auto, so every block renders dir="auto".
  Because serializeEditorContent uses ProseMirror's DOMSerializer, the
  attribute is preserved in the sent HTML — so recipients see the same
  direction, not just the author. The editor root also gets dir="auto".
• email-composer: dir="auto" on the subject input and the plain-text body
  textarea, so those fields flip too.

Notes

• Per-block (not whole-editor) detection means a Hebrew paragraph followed by
  an English one each get the right direction.
• Existing dir on pasted/quoted content is preserved (the attribute parses the
  element's existing dir).
• Reading view is unaffected: the message body already renders true-to-life in
  its iframe, where the email's own markup controls direction.

Test plan

• npm run typecheck, npm run build — green
• Type Hebrew/Arabic in the rich editor → that line flips RTL; English stays LTR
• Type a Hebrew subject → subject field flips RTL
• Plain-text mode → RTL detection works in the textarea
• Send an RTL message → received HTML carries dir="auto" (recipient sees RTL)