feat(desktop): IRC-style s/old/new/ self-correction in the composer

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What

Sending a message whose entire body is a well-formed substitution command — s/hullo/hello/ — now edits your most recent message instead of sending literal text. It's a keyboard-only shorthand for the existing right-click "Edit message" gesture.

If I type hullo there!, notice the typo, and send s/u/e/, my previous message becomes hello there!.

How

Pure client-side sugar. The correction is republished through the same kind-40003 edit path a manual edit uses, so other clients (and the relay) just see a normal edit. No relay, schema, or Rust changes.

• selfCorrection.ts (pure, unit-tested) — parses s/old/new/[flags] as literal text, never regex:
  • Delimiter is always / — the canonical IRC/sed s/old/new/ shorthand. (sed permits arbitrary delimiters only to avoid escaping the delimiter inside the pattern; our \/ escaping already covers that, so alternate delimiters would only widen the accidental-trigger surface for no gain.)
  • \-escaping of the delimiter and backslash, so s/\//\/\//g replaces every / with //.
  • trailing delimiter optional when there are no flags (s/u/e).
  • flags: g (all occurrences) and i (case-insensitive).
  • literal matching — s/*/x replaces the first literal *; . is a literal dot, not a wildcard; $1/\d in the replacement are inserted verbatim.
• selfCorrectionEdit.ts (pure, unit-tested) — resolves a command against the timeline: finds the author's most recent editable message (same eligibility as ArrowUp-to-edit — own, non-system, not pending) and rebuilds the edit through the existing imeta/emoji helpers, so attachments and custom emoji are preserved.
• useSelfCorrectingSend — intercepts at the send boundary in ChannelPane. A valid command edits the previous message; anything that doesn't parse, or whose pattern isn't present in your last message, falls through and sends literally. Drafts that carry their own attachments are treated as ordinary sends (then s/a/b/ is a caption).

Accidental-trigger safety

Three independent guards keep ordinary prose from being eaten:

1. The message must start s/ — prose like s3 bucket, s: notes, s|foo never qualifies.
2. The entire body must be a well-formed command.
3. The pattern must actually occur in your previous message — otherwise it just sends as text, never a silent edit.

Behavior notes

• A typo-fix correction intentionally emits no new mention tags — it re-wakes nobody, and the corrected body keeps the original mention tokens so existing mentions still render. (A substitution that introduces a brand-new @mention won't notify that user; acceptable for a quick-correction affordance.)
• Scoped to the main timeline. Thread-reply correction is a natural follow-up.

Testing

• Unit tests across selfCorrection (grammar/apply — incl. s/u/e, s/\//\/\//g, slash-only enforcement, and a dedicated literal-not-regex proof) and selfCorrectionEdit (target selection, fallthrough, media/emoji preservation).
• pnpm typecheck, biome, file-size guard, and the full desktop unit suite (3860 tests) pass locally.

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🐝 Built by Bumble in Buzz, from a conversation in #test-swesterman.

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🤖 Pushed a refactor commit (12c7e5ae8) that shrinks this PR in response to feedback that it was large for the scope. No behaviour change — still main-composer-only, literal-not-regex, media/emoji preserved.

What shrank the surface (and the future-breakage risk):

• Removed the new onEditSaveById handler + prop that threaded through ChannelScreen → ChannelPane.types → ChannelPane → useChannelPaneHandlers. Instead the existing handleEditSave/onEditSave gains an optional explicit eventId — self-correction now rides the edit path already in place. Zero new cross-component props.
• Reuses ChannelPane's existing findLastOwnEditable instead of a byte-for-byte duplicate, so "what counts as an editable message" has one definition shared with ArrowUp-to-edit.
• Merged selfCorrectionEdit.ts into selfCorrection.ts; folded the two test files into one.

Net: 9 files → 6, 736 → 595 lines. The remaining bulk is the parser + its tests (the escaping / optional-slash / literal-not-regex grammar); the actual wiring is now ~80 lines. Full desktop suite green (3855 tests), typecheck + biome + file-size/px/pubkey guards clean.