* Add adjustable text size setting to interface
Introduces a user-configurable text size (scale) setting, accessible via a slider in the interface settings. Updates CSS and Sidebar chat item components to respect the new --app-text-scale variable, and persists the setting in the store. Adds related i18n strings and ensures the text scale is applied globally and clamped to allowed values.
* Refactor text scale logic into utility module
Moved all text scale related constants and functions from components and stores into a new utility module (src/lib/utils/text-scale.ts). Updated imports and usage in Interface.svelte and index.ts to use the new module, improving code organization and reusability.
* Adjust sidebar chat scaling without extra classes
keep sidebar markup using existing Tailwind utility classes so chat items render identically pre-feature
move all text-scale sizing into app.css under the #sidebar-chat-item selectors
change the root font-size multiplier to use 1rem instead of an explicit 16px so browser/user preferences propagate
* Update Switch.svelte
Adjust toggles from fixed pixel to rem to scale with the text size
* Update Interface.svelte
Updated label from 'Text Scale' to 'UI Scale'.
Added padding around slider
* Update app.css
Added comments
