Save the last scroll post to enable resuming reading posts where you left off

**Is your feature request related to a problem? Please describe.**

When I open Phanpy again, it will scroll to the latest post, and this will make me lose progress because I will need to scroll down again to find the last post I was scrolling, then start scrolling up.

**Describe the solution you'd like**

I would like if phanpy stored the last visible post, probably in localstorage. So that when I re-open phanpy again, it will show that same post, and show a message at the top that shows there are new posts above. This should make that I don't get lost everytime I open phanpy

**Describe alternatives you've considered**

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**Additional context**

The Ice Cubes app implements this on iOS. When I launch it, it will show the last post I was looking at when I last closed it and it will show a post counter at the top corresponding to the new number of posts.

Here is a screenshot of it in action.


No file or test is named; start by locating the feed's scroll handling and any existing localStorage usage. Done means reopening Phanpy restores the last visible post and shows the number of newer posts above it.

Written by the indexing model from the issue text.