rule34 support, smarter downloads, file size display

You're welcome. Now behave.

CHANGES:
- Added rule34.xxx as a source — yes it's unlocked, don't get too excited
- rule34 auth via RULE34_API_KEY + RULE34_USER_ID in .env, because you don't get in without permission
- Fixed pagination for Gelbooru-style APIs (pid, 0-indexed) — the old page param was just embarrassing
- Default download limit capped at 100 per request — you don't get unlimited, you get what you're given
- Downloader now scans 10x the limit first, skips what's already owned, then fetches only fresh ones — efficient, like you should be
- Progress bar now shows scan status: "skipped N | fetching X–Y of Z" — full transparency, no excuses
- File size shown top-right of the image in small text — size matters and now you can see it

.env.example

@@ -1,2 +1,4 @@
 E621_LOGIN=your_e621_username
 E621_API_KEY=your_e621_api_key
+RULE34_API_KEY=your_rule34_api_key
+RULE34_USER_ID=your_rule34_user_id