feat(services): add Iframe service for embedding pages in cards

[Sportsocke522]

Description

Adds a new lightweight Iframe service extending the built-in Generic card.

• Renders only the content area (title/subtitle/header are hidden) and fills the card with an <iframe>.
• Height can be controlled by:
  • rows (number of card-heights, default 2)
  • optional base height per row via unit / rowUnit (default 85px, accepts any CSS length), or globally via CSS variable --h-row-unit
  • height (CSS length; hard override of rows)
• No new dependencies. No breaking changes.

Props

• src (required) – URL to embed
• rows (optional, default 2)
• unit / rowUnit (optional, default 85px)

Note: Some sites may block embedding via CSP or X-Frame-Options. This is documented.

Iframe (service)

Embed a web page inside a card.

Config

- type: Iframe
  src: "https://example.com/embed"
  rows: 3         # optional, default 2
  # unit: 90px    # optional per-item base height; default 85px
  # height: 520px # optional hard override (CSS length)

- type: Iframe
    src: https://example.com/embed
    rows: 3
    unit: 100px 

Fixes # (issue)

Type of change

• New feature (non-breaking change which adds functionality)
• Bug fix
• Breaking change
• Refactoring

Checklist:

• I've read & comply with the contributing guidelines
• I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• I have made corresponding changes to the documentation (README.md) – see snippet below.
• I've checked my modifications for any breaking changes, especially in the config.yml file.