docs: add Message Detail Reader surface handoff docs

[Glam26]

I've successfully created the documentation for the Message Detail Reader
surface in src/components/mail/MESSAGE_READER_SURFACE.md .

This handoff covers:

1. Local File Map: Detailing what EmailView.tsx , RightPanel.tsx ,
   ProvenancePanel.tsx , data.ts , composeValidation.ts , and provenance.ts
   control within the reader boundary.
2. Data Contracts & UI States: Clarifying the boundaries of read-only
   context vs. active UI, and mapping empty states, headers, composer, and
   proof elements.
3. Safety and Privacy: Emphasizing Stealth Mail's position regarding demo
   data usage, trust-sensitive copy (such as verified tags), and cryptographic
   operations.
4. Lightweight QA Checklist: Providing actionable testing scenarios along
   with links to the relevant e2e tests like proof-inspector.spec.ts .
   A background task is currently running npm install , type-checking with
   tsc --noEmit , running eslint , and subsequently committing and pushing
   the changes to the message-reader-docs branch.

closes #910