## Summary

Adds two user-facing settings that let the markdown preview render with
a font family and theme independent from the editor. Both are optional
and fall back to the editor defaults when unset.

```json
{
  "markdown_preview_font_family": "Georgia",
  "markdown_preview_theme": "Solarized Light"
}
```

## Details

- New `MarkdownFont::Preview` variant, used only by the preview surface.
- New `MarkdownStyle::themed_with_overrides()` that accepts explicit
`ThemeColors` and `SyntaxTheme` so the preview can render with a theme
other than the active editor theme. Existing `themed()` callers are
unchanged.
- The preview pane background adopts the chosen theme's
`editor_background`.
- Follows the pattern already used by `agent_ui_font_size` /
`agent_buffer_font_size`.

## Scope

Sizing-related changes (`markdown_preview_font_size`,
`markdown_preview_line_height`, typography tweaks, responsive max-width
container) were in an earlier revision of this PR and have been removed
per review feedback — they interact with the Cmd+/- zoom behavior in
ways that still need design work, and will land in a follow-up.

## Files Changed

- `crates/settings_content/src/theme.rs` — schema
- `crates/theme_settings/src/settings.rs` — runtime + accessor
- `crates/markdown/src/markdown.rs` — `MarkdownFont::Preview` +
`themed_with_overrides()`
- `crates/markdown_preview/src/markdown_preview_view.rs` — theme
resolution, style selection, background
- `crates/markdown_preview/Cargo.toml` — `theme` dependency
- `crates/settings/src/vscode_import.rs` — new fields in struct literal

Release Notes:

- Added `markdown_preview_font_family` and `markdown_preview_theme`
settings to customize the markdown preview independently from the
editor.

Co-authored-by: Chris Biscardi <chris@christopherbiscardi.com>