Add raw text diff to end-of-edit notification emails

[Copilot]

This plugin notifies subscribers when editing starts/ends, but end-of-edit emails previously only included a pad link. This change adds a raw, line-based diff so recipients can quickly see what changed without opening the pad.

• Diff content in end-of-edit emails

  • Capture pad text at the beginning of an edit session.
  • On stale/end notification, compare start vs final text.
  • Append a raw +/- line diff to the “finished editing” email body.

• Diff generation helper

  • Added textDiff.js to produce deterministic line-based diffs.
  • Uses LCS for normal-sized inputs.
  • Includes fallback limits for large inputs to avoid excessive CPU/memory and oversized email content.

• Session/race handling

  • Timer/session state now carries startText.
  • End notification consumes captured start state explicitly, avoiding cross-session/missing-state edge cases.

• Regression coverage

  • Added backend tests for unchanged, added, removed, replaced, and large-input fallback diff behavior.

// update.js (end notification flow)
const startText = timer ? timer.startText : null;
delete timers[padId];
await notifyEnd(padId, startText);

// notifyEnd()
const {text = ''} = await API.getText(padId);
const diffText = textDiff(startText, text);
// ...append diffText to email body when non-empty