docs(demos) Pad the union-trim crop for breathing room

why: The tight union-trim crop left only a 14px margin, so the content
felt cramped against the edges. Restore a 28px margin on all sides —
comfortable, and just past the terminal's native ~20px padding.

what:
- Re-render the D variant with -border 28 (382x181 -> 410x209)
- Update the comparison page's dimensions and padding note

Author: tony

docs/_static/demos/crop-comparison/sync-imtrim-opt.gif

@@ -37,11 +37,13 @@ Coalescing also unoptimised the file to 360K.
 GIF frames are intra-coded; re-encoding to H.264 yielded no usable motion
 vectors, so the motion-based detector had nothing to work from.
 
-## D · ImageMagick union-trim (full crop) — ✅ 382×181 · 68K
+## D · ImageMagick union-trim (full crop) — ✅ 410×209 · 68K
 
-`-coalesce` → clone/flatten → `-fuzz 8% -trim` → viewport crop → `+14px`
-margin → `-layers Optimize`. Removes **all** dead space (horizontal and
-vertical). Tightest result, but drops the wide-terminal framing.
+`-coalesce` → clone/flatten → `-fuzz 8% -trim` → viewport crop →
+`-border 28` → `-layers Optimize`. Removes the horizontal and vertical
+dead space, then restores a **28px** margin on all sides so the content
+isn't cramped (`-border` fills every side with the terminal background;
+dial it via the value — 20px matches the terminal's native padding).
 
 ```{image} _static/demos/crop-comparison/sync-imtrim-opt.gif
 :alt: ImageMagick union-trim, cropped to content with margin
@@ -63,7 +65,7 @@ full terminal width so it still reads as a terminal.
 | A | ffmpeg cropdetect (black) | 1174×202 | ~0% | ❌ bg above threshold |
 | B | ImageMagick TrimBounds | 1175×202, 360K | 0% | ❌ + bloat |
 | C | ffmpeg cropdetect mvedges | — | — | ❌ no motion vectors |
-| D | ImageMagick union-trim | 382×181 | ~72% | ✅ tightest |
+| D | ImageMagick union-trim | 410×209 | ~64% | ✅ recommended |
 | E | vertical-only ffmpeg crop | 1175×181 | ~10% (vertical) | ✅ keeps terminal look |
 
 The only reliable spatial cropper for a terminal GIF is the **ImageMagick