fix(desktop): fold kind 40003 edit content into forum posts and comments

[BradGroux]

Summary

Desktop already fetches kind:40003 edit events alongside forum posts and thread replies, but only uses them for link-preview suppression. The edit content is never applied to the original message, so users always see the original text even after a successful edit — the relay has the new content, the CLI thread view shows it, but Desktop discards it.

Added fold_edit_content, which finds the latest valid 40003 edit for a given original event and returns its content. A valid edit must be signed by the original author or the author's verified agent owner (same authorization model as the existing link_preview_suppression_targets path). Applied in both get_forum_posts and get_forum_thread so edited content shows in both the channel list and the thread view.

This does not add an edit affordance in the UI — it fixes the data layer so that edits made via CLI or relay are visible in Desktop. The UI edit button is a separate enhancement.

Related issue

Closes #6433

Testing

Six new unit tests in forum.rs:

• fold_edit_content_applies_author_edit — author signs an edit, content is replaced.
• fold_edit_content_applies_owner_edit — verified agent owner signs an edit, content is replaced.
• fold_edit_content_rejects_unauthorized_signer — edit from a random key is ignored.
• fold_edit_content_latest_edit_wins — among two valid edits, the higher created_at wins.
• fold_edit_content_ignores_edit_targeting_different_event — edit with an e tag pointing at a different event is ignored.
• fold_edit_content_no_edits_returns_none — no edits means the original content is used.

The existing suppression_targets_accepts_author_and_verified_owner_only test is unchanged. Cannot run the full cargo test suite from this host (no Rust toolchain). The build and test gate will be verified on CI.