Fix bugs and code quality issues from code review

[antfred]

Summary

• Bug fix: `rename` crashed by calling `find_feed` after the folder was already moved
• Bug fix: `episode_too_old` compared UTC `now` against local-time `fromtimestamp`
• Bug fix: `download_single` raised `KeyError` when server omits `Content-Length` header
• Bug fix: `pretty_print_feeds` raised `IndexError` on feeds with no episodes
• Reliability: Replaced bare `except` with specific `(KeyError, IndexError)` in `download_single`
• Refactor: Extracted `_make_feed_folder` to eliminate duplicated folder-creation logic
• Performance: Replaced O(n²) nested loop in `update_feed` with O(1) set-based deduplication
• Cleanup: Removed unused `mimetypes` list; removed redundant `hasattr` check
• Fix: Added missing `exit(0)` after `rename` command in `main`

Test plan

• `podfox import ` works with and without a shortname
• `podfox update` adds new episodes without duplicates
• `podfox download` works when server omits `Content-Length`
• `podfox rename ` renames correctly and updates `feed.json`
• `podfox feeds` displays correctly for feeds with zero downloaded episodes
• `podfox prune` age filtering is consistent regardless of local timezone