refactor: introduce a Feature component for the code-and-text rows

[magnus-madsen]

Summary

• Adds src/components/Feature.astro: one row of the front-page feature tour — a title, the Flix source as code, a codeLeft boolean for which side the snippet sits on, and the prose slotted in as plain <p>s. A named code slot takes anything that is not a single Flix snippet (the manifest row's two PlainSnippets, the test row's output under its code).
• Rewrites the 21 feature rows in src/pages/index.astro to use it (about 500 lines fewer). The hero row and the full-width Datalog row are a different shape and are left as they were.
• One rule in global.css (.feature-text > :last-child) gives plain paragraphs the trailing-margin trim Bootstrap only gives .card-text, so callers no longer need that class on every paragraph.

Behaviour

Desktop rendering is unchanged. codeLeft is order-md-first on the code column and the text is always first in the markup, so on phones every row now reads heading → text → code; the eleven code-left rows used to show a bare snippet above its own heading. If a pixel-for-pixel no-op on mobile is preferred, the component can emit the columns in swapped DOM order instead.

Verification

• astro check clean, astro build succeeds.
• Built dist/index.html before and after and compared each feature row's two columns as an unordered pair (whitespace collapsed, the three known class changes stripped): all 21 rows identical, rest of the page identical modulo the CSS hash, 11 rows carry order-md-first.
• The prose was reflowed for the shallower indent with care not to break a line in the whitespace touching an inline element, since the Astro compiler drops the space there (the reason for the <code\n> style, cf. fix: restore spaces lost between inline elements #212). The check above caught one such break; it is fixed.

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