Make Settings window resizable, responsive scroll area, and add sizing tests

[multiplex55]

Motivation

• Improve usability of the Settings dialog so it can be resized taller and not collapse to an unusable height.
• Remove the hard-coded 300px scroll cap so window stretching increases visible settings content.
• Keep footer actions (Save) accessible regardless of scrollable content height.
• Add unit tests to guard sizing behavior and preserve existing static-position normalization invariants.

Description

• Added sizing constants and helpers: SETTINGS_WINDOW_DEFAULT_SIZE, SETTINGS_WINDOW_MIN_HEIGHT, SETTINGS_CONTENT_MIN_HEIGHT, and helper functions settings_window_default_size, settings_window_min_height, and settings_content_height.
• Made the settings window explicitly resizable and larger by default via .resizable(true), .default_size(...), and .min_height(...) on egui::Window::new("Settings").
• Replaced the fixed egui::ScrollArea::vertical().max_height(300.0) with a responsive cap based on ui.available_height() and clamped via settings_content_height(...) so the scroll area grows with the window while preserving scrolling.
• Kept the Save action reachable by moving the Save button outside the ScrollArea into a fixed footer row and extracted the save flow into a new save_settings function to preserve original save behavior while simplifying the UI code.
• Added unit tests in the #[cfg(test)] module to validate sizing defaults (settings_window_sizing_policy_uses_expected_defaults), the content-height floor (settings_content_height_has_sane_floor), and that to_settings preserves static-position behavior when follow_mouse is disabled (to_settings_preserves_static_values_when_follow_mouse_disabled).

Testing

• Ran code formatting with cargo fmt -- src/settings_editor.rs, which completed successfully.
• Attempted to run cargo test -q settings_editor to execute the new sizing tests, but the test run failed in this environment due to a transitive dependency (alsa-sys) requiring the system alsa pkg-config file (alsa.pc) which is not available, preventing the build and test execution.
• The new unit tests are present and designed to run in CI or a local environment with standard system dependencies installed.

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Codex Task