refactor: de-duplicate bundled auto-editor binary sync in main.py (#7)

[rsmxingu]

Problem

In the frozen-bundle re-entry path (python -m auto_editor), the "copy bundled binary into the writable cache" logic — exists check, makedirs, stale-size check, copy2, chmod — was written twice: once to pre-populate the cache, and again inside the download_binary monkeypatch. The two copies could drift independently.

Change

Extract a single _sync_bundled_binary() helper that copies the binary when the target is missing/stale and returns its Path (or None when no bundled binary exists). Both call sites use it:

• pre-population calls it best-effort (errors still swallowed — the monkeypatch is the real fallback);
• _use_bundled_binary() raises FileNotFoundError when it returns None.

Behavior is unchanged.

Testing

• python3 -m py_compile main.py passes.
• This only touches the python -m auto_editor re-entry block, which is guarded by the argv check and never executes during import or the unit tests, so tests/test_main.py (start/cancel/on_closing) is unaffected.
• tests/test_main.py itself requires tkinter (via import ui), unavailable in my sandbox, so I could not execute it here. Please run python3 -m unittest tests.test_main in a tkinter environment to confirm.

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