Feat:Add configurable duration for Slideshow

### Describe the feature

### Problem

The slideshow feature currently auto-advances at a fixed **3s interval**, with no way to control the pace.

### What I checked

- No hidden setting/option exists in the current UI for this
- Searched open and closed issues for "slideshow" — found related ones (looping, story mode, etc.) but none covering configurable duration

### Proposed Change

Add a duration picker (`2s` / `5s` / `10s`) that appears when the user clicks **Slideshow**. User selects a duration, then clicks **Start** to begin the slideshow at that pace.

### Tech Notes

Frontend-only change — touches `useSlideshow.ts` and `MediaViewControls.tsx`.

I've already built and tested a working version locally — confirmed all three durations work correctly. Would like to work on this, happy to open a PR once approved.

### Add ScreenShots

### Record

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- [x] I want to work on this issue