fix: don't save settings immediately after login

The save-settings $effect was firing on the initial data load because
settingsApplied was set to true synchronously in the apply effect, then
currentTheme/fontFamily/fontSize were written in the same tick — causing
the save effect to immediately fire with uninitialized default values
(theme: "", fontFamily: "", fontSize: 0), producing a 400 error.

- Add settingsDirty flag, set via setTimeout(0) after initial apply so
  the save effect is blocked for the first load and only runs on real
  user-driven changes
- Also accept empty string / 0 as 'not provided' in PUT /api/settings
  validation as a defensive backstop

ui/src/routes/api/settings/+server.ts

@@ -43,27 +43,27 @@ export const PUT: RequestHandler = async ({ request, locals }) => {
 		error(400, 'Invalid body — expected { autoNext, voice, speed }');
 	}
 
-	// theme is optional — if provided it must be a known value
+	// theme is optional — if provided (and non-empty) it must be a known value
 	const validThemes = ['amber', 'slate', 'rose', 'light', 'light-slate', 'light-rose'];
-	if (body.theme !== undefined && !validThemes.includes(body.theme)) {
+	if (body.theme !== undefined && body.theme !== '' && !validThemes.includes(body.theme)) {
 		error(400, `Invalid theme — must be one of: ${validThemes.join(', ')}`);
 	}
 
-	// locale is optional — if provided it must be a known value
+	// locale is optional — if provided (and non-empty) it must be a known value
 	const validLocales = ['en', 'ru', 'id', 'pt', 'fr'];
-	if (body.locale !== undefined && !validLocales.includes(body.locale)) {
+	if (body.locale !== undefined && body.locale !== '' && !validLocales.includes(body.locale)) {
 		error(400, `Invalid locale — must be one of: ${validLocales.join(', ')}`);
 	}
 
-	// fontFamily is optional — if provided it must be a known value
+	// fontFamily is optional — if provided (and non-empty) it must be a known value
 	const validFontFamilies = ['system', 'serif', 'mono'];
-	if (body.fontFamily !== undefined && !validFontFamilies.includes(body.fontFamily)) {
+	if (body.fontFamily !== undefined && body.fontFamily !== '' && !validFontFamilies.includes(body.fontFamily)) {
 		error(400, `Invalid fontFamily — must be one of: ${validFontFamilies.join(', ')}`);
 	}
 
-	// fontSize is optional — if provided it must be one of the valid steps
+	// fontSize is optional — if provided (and non-zero) it must be one of the valid steps
 	const validFontSizes = [0.9, 1.0, 1.15, 1.3];
-	if (body.fontSize !== undefined && !validFontSizes.includes(body.fontSize)) {
+	if (body.fontSize !== undefined && body.fontSize !== 0 && !validFontSizes.includes(body.fontSize)) {
 		error(400, `Invalid fontSize — must be one of: ${validFontSizes.join(', ')}`);
 	}