Feat: Add toggle button to collapse sidebar and give more space for photos

### Describe the feature

**The Issue:**
Right now, the sidebar is always visible, which takes up screen space that could be used to show more photos — especially on smaller screens or in fullscreen mode.

**Suggestion**
Add a collapse/expand toggle button (like the chevron in Notion or VS Code) at the top of the sidebar so users can:

**Hide the sidebar to see more images at once**
- Keep it accessible via icons + hover tooltips when collapsed
- Have their preference saved between app restarts

**Why it’s a good fit**
> Better gallery experience: More space = bigger thumbnails, less scrolling
Low effort: Uses existing tools (Lucide icons, Radix tooltips, Tauri store)
No breaking changes: Pure frontend tweak (~50 lines)
**User control:** People can choose what works for their screen
This is a small change with a noticeable improvement in daily use. Would you be open to this? I’d be happy to raise a PR!

### Add ScreenShots

https://github.com/user-attachments/assets/42792131-a33e-4b66-9566-9103805526a0

### Record

- [x] I agree to follow this project's Code of Conduct
- [x] I want to work on this issue