feat(settings): add persistent UI scale control (#51)

[Horuse]

What does this PR do?

Adds an Interface scale control to Settings -> Appearance (50-200%, with Reset). The value persists across launches and scales the whole UI through the Tauri webview zoom.

Also fixes two issues that zoom exposed: the macOS header now keeps aligned with the native traffic lights at any scale, and the eye icon in the password input no longer blows up.

Why is this the right approach?

Webview zoom scales everything at once with no per-component changes. No new dependencies, only the core:webview:allow-set-webview-zoom permission.

Checklist

• Diff is limited to the change - no unrelated edits
• bun run check passes
• cargo check --manifest-path src-tauri/Cargo.toml passes
• bun run format leaves the tree clean
• Generated TS types are committed with the Rust change (if any) - N/A
• No new dependency without a reason in the PR description
• I read the RT audio path section of docs/CONCEPT.md and confirmed this change adds no allocations, locks, or syscalls to cpal / SCK callbacks or DspWorker::run - frontend only

Platform coverage

• Developed on: macOS
• Tested on: macOS
• What I did to test: checked scales 50-200%, header alignment and password inputs.
• Not tested: Linux / Windows

Header compensation applies only on macOS (native titlebar).

Related

• Closes [feature] UI and Text size customization #51