feat: add interactive prompt for video generation (#298)

## Summary

After finishing a recording, t-rec now prompts users to generate an MP4
video in addition to the GIF. This provides a convenient way to get both
formats without needing to re-record or remember to pass `--video`
upfront.

The prompt runs in a background thread, so users can answer while the
GIF is being generated - no wasted waiting time.

### User Experience

After the GIF generation message appears, users see:
```
🎉 🚀 Generating t-rec.gif

🎬 Also generate MP4 video? (y/n) ›
(auto-skip in 15s)
```

- Press `y` to generate MP4 (no Enter required)
- Press `n` to skip (no Enter required)
- Wait 15 seconds to auto-skip

### When the prompt is skipped

- **Quiet mode** (`-q`): No prompt, respects automation needs
- **Non-interactive** (piped input, CI): Returns "no" automatically
- **Already requested video**: If `--video` or `--video-only` was used,
video generates without prompting

### Implementation

- Uses `dialoguer` crate for cross-platform terminal input handling
- Prompt runs in background thread while GIF generates
- 15-second countdown displayed below the prompt

Closes #219

## Test plan

- [x] Run `t-rec`, wait for prompt, press `y` → video generated
- [x] Run `t-rec`, wait for prompt, press `n` → no video  
- [x] Run `t-rec`, wait 15 seconds → timeout, no video
- [x] Run `t-rec --video` → no prompt, video generated
- [x] Run `t-rec --video-only` → no prompt, only video
- [x] Run `t-rec -q` → no prompt (quiet mode)
- [ ] Run `echo "" | t-rec` → no prompt (non-interactive)

---------

Signed-off-by: Sven Kanoldt <sven@d34dl0ck.me>

Author: sassman

Cargo.lock

@@ -29,6 +29,7 @@ simplerand = "1.6"
 serde = { version = "1.0", features = ["derive"] }
 toml = "0.8"
 dirs = "5.0"
+dialoguer = { version = "0.11", default-features = false }
 
 [dependencies.clap]
 version = "4.5"

Cargo.toml

@@ -279,6 +279,29 @@ t-rec --fps 15
 
 **Note:** Higher framerates produce larger files. The default 4 fps is recommended for most use cases.
 
+### Video Generation
+
+After recording, t-rec will ask if you also want to generate an MP4 video:
+
+```
+🎉 🚀 Generating t-rec.gif
+
+🎬 Also generate MP4 video? (y/n) ›
+(auto-skip in 15s)
+```
+
+- Press `y` to generate both GIF and MP4
+- Press `n` or wait 15 seconds to skip
+
+To always generate video without being asked, use the `--video` flag:
+
+```sh
+t-rec --video          # Generate both GIF and MP4
+t-rec --video-only     # Generate only MP4, no GIF
+```
+
+The prompt is skipped in quiet mode (`-q`) or non-interactive environments.
+
 ### Disable idle detection & optimization
 
 If you are not happy with the idle detection and optimization, you can disable it with the `-n` or `--natural` parameter.