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/lib/paraglide/messages.js

@@ -1,4 +1,2 @@
 /* eslint-disable */
-export * from './messages/_index.js'
-// enabling auto-import by exposing all messages as m
-export * as m from './messages/_index.js'
+export * from './messages/_index.js'