Replace bare assertions with proper error handling; add crypto tests

[offbyonebit]

Summary

• Bug fix: 5 bare assert statements used as production error guards replaced with explicit if/raise RuntimeError or logged early returns — these were silently no-ops under python -O and would produce unhelpful AssertionError tracebacks otherwise
• New tests: tests/test_crypto.py adds 18 tests for the encryption module (crypto.py), which had zero dedicated coverage despite being security-critical
• All 61 tests pass

Changes

Assertion fixes

File	Location	Old	New
main.py:155	after Syncthing start	assert client is not None	raise RuntimeError(...)
main.py:312	_accept_device()	assert client is not None	log + early return
syncthing.py:715	_spawn()	assert self._binary is not None	raise RuntimeError(...)
ui.py:90	_read_events()	assert proc.stdout is not None	raise RuntimeError(...)
ui.py:1023	_finish_update_check()	assert info is not None	if info is None: return

New crypto tests cover

• Encrypt/decrypt round-trip (text, binary, empty, unicode passphrase)
• Wrong passphrase returns None
• V1 produces magic header and random salt per call
• V0 legacy format backward compatibility
• Truncated, garbage, and partial-magic input
• is_encrypted() for v0, v1, plaintext, and empty input

Test plan

• pytest tests/test_crypto.py — 18 new tests pass
• pytest tests/ (excluding integration) — all 61 tests pass

https://claude.ai/code/session_01BULAPFvuh92QBWZboAuSL2

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