Add meme history to track previously generated memes

I keep losing track of memes I made earlier. Would be great to see my history.

**Problem:**
- No way to see previously generated memes
- Have to regenerate same memes again
- Can't share old memes with friends

**What I want:**
- History page showing all my generated memes
- Ability to re-download old memes
- Clear history option
- Show creation date and template used

This would save so much time when I want to find a meme I made last week.

The issue names no files or tests. Start by locating the meme-generation entry point and determine how generated memes and their metadata can be persisted; done means a history page lists created memes with dates and templates, supports re-download, and offers clearing history.

Written by the indexing model from the issue text.