feat(settings): add appearance preferences

[kevincador]

Important

Proof of concept for discussion.
This PR is intended to evaluate the interaction model and visual direction. Do not merge it in its current form.

Summary

Add three persisted appearance preferences that let people simplify and constrain the interface:

• Reduce visual effects
• Reduce width
• Center drawers

The settings are available from General settings and from an Appearance drawer opened through the footer.

Changes

• Reduce decorative imagery, gradients, shadows, blur, animation, and other visual effects across shared components.
• Replace removed elevation with consistent, theme-aware borders and higher-contrast flat surfaces.
• Render spoilers with a redacted treatment.
• Limit page content to 1,280px while accounting for navigation, sidebars, placeholders, and horizontal rail calculations.
• Center and widen drawers with a dimmed underlay, adjusted motion, and live setting previews.
• Preserve drawer state, focus, and scroll position when changing the setting.
• Keep the three-dot contrast gradient on image-backed cards while removing it from plain surfaces.
• Add footer access using the Material Format Paint icon and align the Preview Features icon with Material Experiment.
• Correct horizontal rail sizing so complete cards fit within their content containers.

Design sync questions

• Are the three settings named and described clearly?
• Is the scope of Reduce visual effects appropriate, or should some treatments become separate preferences?
• Does the 1,280px content cap produce the right density across sidebar states?
• Are the centered drawer width, motion, and underlay treatment appropriate?
• Are the flat borders and card backgrounds sufficiently distinct in both themes?
• Are the footer entry points discoverable without adding too much visual weight?

Risk and follow-up

• The semantic style-token changes affect many shared components and need broad visual review.
• Drawer portal behavior should receive additional keyboard and stacked-drawer testing.
• Rail sizing should be checked across Safari, Firefox, RTL, touch, and coarse-pointer layouts.

Screenshots

The footer

The Appearance drawer

With all settings enabled (home, media, drawers,...)