Improved API key persistence and config recovery

[karamouche]

Summary

• Fail safely when the existing config cannot be read or parsed, preventing API keys and other settings from being overwritten by defaults.
• Surface config-load failures separately from a missing API key and provide Retry and Open logs actions.
• Add an explicit two-step settings reset that keeps a timestamped backup, refuses readable configs, and restores the original if reset fails.
• Add redacted config diagnostics without logging API key contents.

Test plan

• npm run test
• cargo clippy --manifest-path src-tauri/Cargo.toml --all-targets -- -D warnings
• npm run build
• Manually verify a saved API key survives a full quit and relaunch.
• Manually corrupt config.json and verify Retry/Cancel do not reset it, then confirm Reset all settings creates defaults and retains a timestamped backup.