feat: persist runtime UI preferences separately from config

[devmobasa]

Summary

Should resolve #293

Adds durable runtime UI state for direct overlay customizations without rewriting unrelated authored configuration.

Wayscriber now stores these preferences in $XDG_DATA_HOME/wayscriber/runtime-ui.toml:

• top and side toolbar pin/minimized state;
• active side pane and collapsed sections;
• individual toolbar item visibility and order; and
• per-board pin state.

config.toml remains the authored source for configured defaults, while session files remain responsible for annotation data.

Reliability and recovery

• Applies configured values as seeds and reconciles retained runtime overrides after config or session reloads.
• Uses conditional writes tied to the inspected file and parent-directory identity.
• Rejects symlinked or externally replaced targets.
• Preserves invalid or newer-format files before an explicit reset.
• Treats newer formats as read-only rather than downgrading them.
• Keeps the app usable when runtime persistence is unavailable; changes then apply for the current run only.
• Rolls back rejected mutations and only confirms persistence after the write outcome is known.
• Saves annotation session state before draining runtime UI persistence during shutdown.
• Keeps board runtime pins separate from board values written back to config.toml.

The configurator continues to edit configured defaults only.