fix: avoid false delete events during atomic saves

[woodrow]

Problem

Open Markdown can mis-handle editor save workflows that temporarily unlink a file (for example Vim-style atomic/truncate+rewrite saves). The app currently treats unlink as an immediate delete, clears the open document, and shows a deleted-file error even when the file is recreated moments later.

Solution

Treat delete events as pending and confirm them after a short grace period. If an add or change for the same path arrives before confirmation, cancel the pending delete and process the update normally.

What Changed

• Added FILE_DELETE_CONFIRM_MS (750ms) in shared constants.
• Updated FileWatcherService to:
  • track pending delete timers per file,
  • schedule deletion confirmation on unlink instead of immediate delete,
  • cancel pending delete on add and change,
  • confirm file is still missing before emitting delete,
  • keep watcher entries alive through transient unlink/recreate flows.
• Expanded watcher unit tests to cover:
  • add handler registration,
  • delayed delete confirmation behavior,
  • transient unlink -> add and unlink -> change paths (no delete emitted),
  • recovery-path refresh assertions (change callback still fires),
  • updated multi-file delete test for delayed confirmation.

AI Disclosure

I used AI assistance (OpenAI Codex) to help draft and implement the code changes and tests, and to prepare this PR description. I personally reviewed and validated the final patch and test results, and tested the app works as expected with pnpm start, before opening this PR, though I am not a frontend expert by any means.