Improve GUI startup time

[Josef-Haupt]

Just small improvements sadly nothing major

[Copilot]

Pull request overview

This PR aims to improve GUI startup time by deferring heavyweight imports until they’re actually needed during user interactions, reducing initial module import cost when launching the GUI.

Changes:

• Lazy-load birdnet_analyzer.audio in the single-file GUI tab only when audio operations are triggered.
• In the evaluation GUI tab, defer imports of matplotlib, process_data, and DataProcessor until the related actions run.
• Add from __future__ import annotations and TYPE_CHECKING-guarded imports to keep runtime imports light while preserving type hints.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
birdnet_analyzer/gui/single_file.py	Removes eager audio import and performs local imports in the two callbacks that actually read audio data.
birdnet_analyzer/gui/evaluation.py	Defers heavier evaluation/plotting imports to action handlers; uses postponed annotations + TYPE_CHECKING to avoid runtime type-import costs.

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