fix: guard against font_size=0 collapsing chapter text

- Replace ?? with || when reading font_size so 0 falls back to 1.0
  (affects GET /api/settings, layout.server.ts, +layout.svelte)
- Remove the explicit 'body.fontSize !== 0' exception in PUT /api/settings
  validation so 0 is now correctly rejected as an invalid font size
- Add add_index helper + idx_chapters_idx_slug_number declaration to
  scripts/pb-init-v3.sh (idempotent UNIQUE INDEX on chapters_idx)

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

@@ -18,7 +18,7 @@ export const GET: RequestHandler = async ({ locals }) => {
 			theme: settings?.theme ?? 'amber',
 			locale: settings?.locale ?? 'en',
 			fontFamily: settings?.font_family ?? 'system',
-			fontSize: settings?.font_size ?? 1.0
+			fontSize: settings?.font_size || 1.0
 		});
 	} catch (e) {
 		log.error('settings', 'GET failed', { err: String(e) });
@@ -61,9 +61,9 @@ export const PUT: RequestHandler = async ({ request, locals }) => {
 		error(400, `Invalid fontFamily — must be one of: ${validFontFamilies.join(', ')}`);
 	}
 
-	// fontSize is optional — if provided (and non-zero) it must be one of the valid steps
+	// fontSize is optional — if provided it must be one of the valid steps (0 is not valid)
 	const validFontSizes = [0.9, 1.0, 1.15, 1.3];
-	if (body.fontSize !== undefined && body.fontSize !== 0 && !validFontSizes.includes(body.fontSize)) {
+	if (body.fontSize !== undefined && !validFontSizes.includes(body.fontSize)) {
 		error(400, `Invalid fontSize — must be one of: ${validFontSizes.join(', ')}`);
 	}