refactor: reduce cognitive complexity and enforce a lint budget

[devmobasa]

Summary

Reduce cognitive complexity across Wayscriber without intentionally changing application behavior.

This refactor:

• splits large config-validation routines into focused normalization and validation stages
• separates app session commands, input actions, presenter transitions, and UI event routing
• decomposes Wayland pointer, keyboard, tablet, render, capture, and event-loop handlers
• breaks toolbar, board-picker, and GTK test scenarios into smaller helpers
• centralizes repeated logic such as submenu anchors, thread joins, and event routing
• enables Clippy's cognitive_complexity lint with a threshold of 20

Motivation

Several functions had accumulated deeply nested branches and multiple responsibilities, making them harder to review, test, and safely modify.

The new structure gives individual operations clearer names and boundaries while preserving the existing control flow.

Behavior

No user-facing behavior changes are intended. This is a maintainability refactor rather than a runtime-performance optimization.

Validation

• ./tools/lint-and-test.sh
• formatting and Clippy checks pass
• all-feature tests pass
• no-default-feature tests pass