feat: Add adjustable text size setting to interface (#19186)

* 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

src/lib/utils/text-scale.ts

@@ -0,0 +1,55 @@
+const TEXT_SCALE_VALUES = [1, 1.1, 1.2, 1.3, 1.4, 1.5] as const;
+
+export type TextScale = (typeof TEXT_SCALE_VALUES)[number];
+
+export const TEXT_SCALE_MIN = TEXT_SCALE_VALUES[0];
+export const TEXT_SCALE_MAX = TEXT_SCALE_VALUES[TEXT_SCALE_VALUES.length - 1];
+
+export const DEFAULT_TEXT_SCALE: TextScale = 1;
+export const DEFAULT_TEXT_SCALE_INDEX = TEXT_SCALE_VALUES.findIndex(
+	(scale) => scale === DEFAULT_TEXT_SCALE
+);
+
+export const getScaleFromIndex = (index: number): TextScale => {
+	if (!Number.isFinite(index)) {
+		return TEXT_SCALE_VALUES[DEFAULT_TEXT_SCALE_INDEX];
+	}
+
+	return TEXT_SCALE_VALUES[index] ?? TEXT_SCALE_VALUES[DEFAULT_TEXT_SCALE_INDEX];
+};
+
+export const findClosestTextScaleIndex = (value: unknown): number => {
+	const numeric = Number(value);
+
+	if (!Number.isFinite(numeric)) {
+		return DEFAULT_TEXT_SCALE_INDEX;
+	}
+
+	let closestIndex = DEFAULT_TEXT_SCALE_INDEX;
+	let smallestDistance = Number.POSITIVE_INFINITY;
+
+	TEXT_SCALE_VALUES.forEach((scale, idx) => {
+		const distance = Math.abs(scale - numeric);
+
+		if (distance < smallestDistance) {
+			closestIndex = idx;
+			smallestDistance = distance;
+		}
+	});
+
+	return closestIndex;
+};
+
+export const resolveTextScale = (value: unknown): TextScale => {
+	return TEXT_SCALE_VALUES[findClosestTextScaleIndex(value)] ?? DEFAULT_TEXT_SCALE;
+};
+
+export const setDocumentTextScale = (scale: TextScale) => {
+	if (typeof document === 'undefined') {
+		return;
+	}
+
+	document.documentElement.style.setProperty('--app-text-scale', scale.toString());
+};
+
+export { TEXT_SCALE_VALUES };