[touch2be]

Summary

• Centralize appearance, language and chat preferences in Settings while keeping the chat sidebar visible.
• Add a quick-preferences popover next to the settings gear at the bottom left: theme, accent, font size, contrast, chat width and language.
• Replace chat-width presets with a persistent 40–100% slider shared by the transcript and prompt; retain compatibility with legacy values.
• Synchronize appearance controls and add English, German and Spanish translations.

Validation

• pnpm run check — formatting, lint and types pass.
• 73 targeted tests across 22 files pass.
• Desktop/mobile browser checks performed for the settings layout and quick-preferences popover.

Dependency

Stacked on #167 (feat/chat-navigation-and-sidebar) to keep this PR focused on the settings changes. Merge the sidebar PR first, then retarget this PR to its destination branch.