feat: add Copy Draft, Markdown, and Word export for AI outputs

Adds three export actions below every Gary assistant response so U.S.
lawyers can move drafts into Word, email, or their own templates in one
click:

- "Copy Draft" - clipboard copy with HTML + Markdown payloads so paste
  into Word preserves headings and bullets; plain-English status text.
- "Download Markdown" - `.md` file with safe filename like
  `contract-review-draft-2026-05-19.md`, wrapped with a short title
  block (matter, Saved Legal Task, date) and an attorney-review
  reminder.
- "Download Word Draft" - `.docx` via the already-installed `docx`
  package; preserves headings, paragraphs, bullets, and simple GFM
  tables; ends with a short footer note.

Helpers live in `frontend/src/lib/exportDraft.ts` and are reused by both
the standalone assistant chat and the per-matter chat. Exports include
only the assistant's prose - internal events, tool calls, IDs, and
annotations are intentionally excluded.

Adds focused tests in `exportDraft.test.ts` covering filename safety,
header content, attorney-review reminder, no API-key/internal-ID
leakage, and clipboard failure messaging.

Updates the U.S. lawyer setup guide with a "Copying or Downloading
Drafts" section, and extends the terminology map.