Respect (or remember) appearance preference on splash screen

**Is your feature request related to a problem? Please describe.**
My appearance preference is set to "auto", to follow my system's appearance, which is set to dark mode most of the time. But when i open Phanpy (installed as a PWA on my phone), the splash screen is in long mode.

This is my splash screen, though my phone is on dark mode and Phanpy's appearance set to auto:

**Describe the solution you'd like**
Phanpy should remember whether it was in light or dark mode last time it was opened, and display the splash screen accordingly. Only then it should check the user's preference and make a change to the app's appearance, if relevant.

Start at the PWA splash-screen startup path and the appearance-preference initialization. Trace how the saved light or dark mode and the system “auto” preference are applied; done means the splash screen matches the remembered appearance before any subsequent appearance update.

Written by the indexing model from the issue text.