Resolves feedback from PR #82 review addressing security vulnerabilities, missing documentation, and code quality issues.

Security: Password exclusion from config files

Passwords are now excluded from both export and import operations to prevent credential leakage via config files:

// Export: password intentionally omitted
fprintf(f, "[network]\n");
fprintf(f, "c64_host=%s\n", c64_host ? c64_host : "");
// Password is intentionally excluded from export for security
fprintf(f, "dns_server_ip=%s\n", dns_server_ip ? dns_server_ip : "");

// Import: password field skipped with info log
} else if (strcmp(key, "c64_password") == 0) {
    // Password is intentionally excluded from import for security.
    // Users must re-enter passwords after importing configuration.
    C64_LOG_INFO("Config import: skipping c64_password (excluded for security)");
}

Passwords remain in OBS settings, protected by OS-level access controls. Users must re-enter passwords after importing configurations.

Documentation: Record A/V Sync feature

Added comprehensive documentation for the "Record A/V Sync" checkbox:

• Purpose: Creates av-sync.csv with detailed A/V synchronization measurements
• Automatically triggers A/V sync test program on Ultimate 64 via REST API
• When to use: Debugging A/V sync issues or contributing test data for bug reports
• Requirements: Ultimate 64 with REST API enabled (optional, works without for basic CSV recording)

Code quality improvements

• Added explanatory comments to empty except clauses in av_sync_csv_validation.py (lines 152, 160) clarifying they skip malformed/non-numeric CSV values
• Removed PLANS.md development artifact from repository

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