feat(settings): add dedicated settings surface

[mattenarle10]

summary

• move non-theme preferences into a dedicated settings overlay
• add bottom-right settings action and Cmd/Ctrl+, shortcut
• keep theme selection in the theme popover
• use smooth sliders that commit to supported named values

validation

• bun test
• bun run build
• cargo fmt --manifest-path src-tauri/Cargo.toml --check
• cargo check --manifest-path src-tauri/Cargo.toml

No release changes included.