feat(swa): add subject tag to notes and replies

[Ryback2501]

Summary

• Add NIP-14 subject lines to text notes: an optional subject on new notes, shown as a header in the feed, and replicated (editable, pre-filled with Re:) when replying.

Changes

Threading helpers (swa/)

• threading.js — getSubject(event) returns the first subject tag value (or ''); adornReplySubject(subject) prepends Re: unless the subject already starts with re: (case-insensitive).
• threading.test.js — unit tests for both helpers (present/absent/first-of-many; empty/plain/already-prefixed).

Compose & reply (swa/)

• index.html — optional subject input (max 80 chars) in the compose panel.
• app.js — the post handler attaches a subject tag when set (clearing the field after posting); onReply takes the reply form's subject and attaches it. Mention #[n] offsets account for the leading subject tag so references stay correct.
• feedView.js — the reply form gains a subject input pre-filled with the Re:-adorned parent subject (editable/clearable), passed through to onReply.

Display (swa/)

• feedView.js — renderEvent and renderReply show a .event-subject header (via .textContent) above the content when a subject is present.
• style.css — .event-subject styling.

Test plan

• Post a note with a subject; confirm it carries a subject tag and shows as a header.
• Post with the subject left blank; confirm no subject tag is added.
• Post a note with both a subject and an @<hex> mention; confirm the mention still resolves (correct tag index).
• Reply to a subjected note; confirm the field is pre-filled with Re: …, is editable/clearable, and the reply carries the edited subject.
• Confirm subject + proof-of-work compose (mining still succeeds with a subject set).