Fix landing icons, add landing styles, and use bundled Book theme

[Sendipad]

Motivation

• Preserve the site base path at https://sendipad.github.io/uph/ and avoid root-domain routing surprises.
• Prevent oversized SVG icons on the landing/home page which made feature/doctype/usecase icons render very large.
• Use the bundled book theme so the site renders with local templates instead of a remote theme.

Description

• Ensured baseURL remains https://sendipad.github.io/uph/ and switched the theme to the local book theme by updating config.toml.
• Removed layout: landing from content.en/_index.md and content.ar/_index.md so the home pages render with the normal home/index templates.
• Added explicit landing-page styles in static/css/landing.css that introduce grid/card layout rules and constrain icon wrapper sizes and nested SVG dimensions for .feature-icon, .doctype-icon, and .usecase-icon.
• Committed the changes as Tighten landing icon sizing and preserve /uph base URL config (HEAD: d8e9ca7).

Testing

• Verified configuration and edits with rg and sed/nl to confirm baseURL and new CSS selectors were added (succeeded).
• Inspected the git diff and committed with git commit to record the change (succeeded).
• Attempted a Playwright navigation/screenshot against a local Hugo preview at http://127.0.0.1:1313/en/, but the local Hugo server did not respond (ERR_EMPTY_RESPONSE), so visual validation could not be captured (failed).
• Attempted git pull --ff-only docs docs to sync the remote docs branch, but the remote docs repository is not configured in this environment (failed).

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Codex Task