Backgrounds: solid, gradient, image, presets, auto

Add `--background ` for the area behind the card (today it's transparent + padding):
- `transparent` (current)
- `#hex` solid
- `gradient:colorA,colorB[,angle]`
- `preset:` (curated, ray.so-style)
- `image:`
- `auto` — subtle gradient derived from the theme background

Refines #2 (custom style).

- [ ] `--background` parser + interactive prompt
- [ ] Gradient + preset rendering
- [ ] Image background (cover/contain)
- [ ] `auto` derivation from `ResolvedTheme.bg`

Start at the CLI handling for the `--background` parser and interactive prompt, then trace the rendering path and the `ResolvedTheme.bg` value. Implement the listed transparent, solid, gradient, preset, image, and auto cases, including cover/contain behavior for images. Done means each checklist item works through the CLI.

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