refactor(styles): consolidate page headers & unify landing section headings

[khsrali]

Draft. Addresses #176 and #163 (style uniformity).

#176 — one shared page-header primitive

The site had 5 near-duplicate page-header implementations with inconsistent markup (<section> vs <header>) and per-file mobile overrides:
.section-header · .eco-hero · .docs-hero · .reg-hero · .blog-header (+ their -sub variants).

• Added a single .page-header / .page-header-sub primitive to global.css (loaded on every page), including the mobile and subtitle-link styles.
• Repointed every sub-page to it as <header class="page-header"> (ecosystem, docs, plugin registry, blog, team, events, use-cases, graph gallery, testimonials, acknowledgements, science).
• Deleted the 5 duplicate blocks and their scattered @media overrides.

#163 — uniform landing section headings

Landing <h2>s were all over the place: sizes 2.5 / 2 / 1.8 / 1.25rem, mixed alignment, and multiple colors (rotating orange/green/blue accent bars, blue text). The citation section also had a solid tinted background that didn't cross-fade with its siblings.

• Added one .section-heading class — 2rem / 700 / --color-text / a single blue (--color-primary) accent bar — applied to every live section heading, with a --left variant for the two carousel headers (news, testimonials) that share a flex row with nav arrows.
• Removed the rotating multi-color bars, the 2.5rem provenance outlier, and the uppercase blue "Supported by".
• Made the landing citation section flow: dropped its tinted background/top-border so it cross-fades like the rest. The acknowledgements variant="panel" Cite is unchanged.

Net −123 lines (mostly removed duplication).

Decisions taken (open to change)

1. Scope — focused on headers + landing Inconsistent styling in landing page #163; deeper token/utility consolidation left as a follow-up.
2. Alignment — news & testimonials stay left-aligned (they sit beside carousel arrows); everything else centered.
3. Accent bars — kept, but a single blue everywhere rather than removed.
4. Cite — made to flow (transparent) rather than kept as a distinct panel.

One thing worth an eye: "Supported by" is now a full section heading (was a small uppercase label) — bigger over the logo strip; easy to tone down.

Verification

• astro build — 161 pages, no errors.
• Reviewed with Playwright at 1440px in light and dark themes: all six landing headings render identically; the two carousel headers are left-aligned with left bars; the cite section is transparent/flowing; ecosystem / plugin-registry / blog page headers are now pixel-identical.

Kept as a draft pending confirmation on the decisions above; will switch the issue links to Closes once approved.

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