feat: Add reader mode

[lunauii]

GuideNH would benefit from having an optional global toggle (likely on the top bar in the screenshot attached) for page content to be squished on the left and right sides to more accurately reflect the reading UX of Wikimedia-style articles.

The following should be present:

• margin-left/margin-right of page content ~20-25% on both sides
• minimum width page contents before force zooming out when going even further (already present, but shouldn't regress while toggled)
• dynamic rescaling of GuideNH components (GameScenes, BlockImages, ...) (also current behaviour, but also shouldn't regress)

[ABKQPO]

try use new action

    # Global content zoom factor for the guide screen. Individual pages can override this with the 'zoom' frontmatter field. Default: 1.0. Range: 0.5 to 3.0. [range: 0.5 ~ 3.0, default: 1.0]
    S:contentZoom=1.0

    # Side margin ratio applied to each side of guide content while using the optional narrow reading mode in full-width layout. Set to 0 to disable. A value of 0.15 leaves about 15% margin on both sides. [range: 0.0 ~ 0.45, default: 0.0]
    S:fullWidthNarrowReadingMarginRatio=0.0