Change PageContentType to a sealed class

[ahmedre]

This allows passing more details to types that need it.

[github-actions]

OLD: app-madani-debug.apk (signature: V1, V2)
NEW: app-madani-debug.apk (signature: V1, V2)

          │           compressed           │           uncompressed           
          ├───────────┬───────────┬────────┼───────────┬───────────┬──────────
 APK      │ old       │ new       │ diff   │ old       │ new       │ diff     
──────────┼───────────┼───────────┼────────┼───────────┼───────────┼──────────
      dex │  12.2 MiB │  12.2 MiB │ +686 B │  31.1 MiB │  31.1 MiB │ +1.5 KiB 
     arsc │   2.1 MiB │   2.1 MiB │    0 B │   2.1 MiB │   2.1 MiB │      0 B 
 manifest │   5.5 KiB │   5.5 KiB │    0 B │  26.5 KiB │  26.5 KiB │      0 B 
      res │   1.6 MiB │   1.6 MiB │    0 B │   1.8 MiB │   1.8 MiB │      0 B 
    asset │ 404.2 KiB │ 404.2 KiB │    0 B │ 678.6 KiB │ 678.6 KiB │      0 B 
    other │ 180.5 KiB │ 180.5 KiB │   +3 B │ 368.8 KiB │ 368.8 KiB │      0 B 
──────────┼───────────┼───────────┼────────┼───────────┼───────────┼──────────
    total │  16.5 MiB │  16.5 MiB │ +689 B │    36 MiB │    36 MiB │ +1.5 KiB