Landing page UX and layout refinement (#146)

[aizuanjeme]

Addresses #146.

Refines the landing page's layout and UX. Previously each section had its own
type scale, container width, and spacing — the page read as five unrelated
blocks. Everything now shares one design language: a common heading scale, one
container width and gutter, full-bleed colour bands, and consistent card
treatment.

Content is unchanged apart from one deviation, noted at the bottom.

Hero

• Responsive type scale (2rem → 6xl) replacing fixed sizing
• Gradient headline treatment, reused as the accent in every section below
• Concentric-ring concept diagram echoing the Payjoin mark, with the protocol's
  benefits radiating from it
• Navbar goes transparent over the hero and reverts on scroll
• Supporter logo marquee in a lighter band at the base

Why Payjoin

• Three feature cards with stroke icons, hover lift, and a radial accent wash
• Section header with uppercase eyebrow, gradient headline, and lead paragraph

Testimonials

• Quote-mark card layout: mark → quote → attribution pinned to the card floor,
  so names line up across the row
• Equal-height cards, replacing content-driven heights that left the row ragged
• Hand-drawn circle and sparkle accents on the heading; smiley doodle in the band
• pauseOnHover on the marquee — these cards hold long text meant to be read
• Mobile fix: cards were 24rem wide, wider than a 375px viewport, so no
  whole card was ever visible. Now viewport-relative below sm.
• Mobile fix: the marquee's default 200px edge fade covered over half a phone
  screen. Now proportional.

Prototype demo

• Aspect-ratio frame (3/4 on phones, 16/10 on desktop) instead of 66vh,
  which changed shape with every viewport height
• Embed sits in a bordered frame rather than floating as a raw iframe
• Pointing hand doodle directing attention into the frame
• Adds loading="lazy" — it's a heavy third-party embed below the fold
• Fixes allowfullscreen → allowFullScreen, which is not a valid React DOM
  prop and was being dropped with a console warning

Closing CTA

• Dropped min-h-screen, which produced a mostly-empty viewport-tall block
• Two stacked <h1>s became a single <h2> — the hero already owns the page's
  h1, so this also fixes the heading hierarchy
• Stepped-path artwork as a backdrop, with a scrim keeping the copy readable
• Newsletter and Discord promoted to CTA buttons matching the hero's

Footer

• Oversized PAYJOIN wordmark that fades into the page edge. Uses SVG
  textLength + lengthAdjust="spacing", so the letters span the full width
  exactly at any viewport by opening the gaps between them — a vw-sized
  heading can only approximate that and overshoots into a horizontal scrollbar.
• Link columns restyled: uppercase labels, muted links, pink hover
• Added as a wrapper swizzle (@theme-original/Footer), not an eject, so
  footer links and copyright stay config-driven in docusaurus.config.ts and
  Docusaurus upgrades won't conflict

Page shell

• Removed max-sm:gap-20, which showed the page background as a bright stripe
  between every section on mobile now that sections paint their own bands

Colours

All from the existing palette (#f75394 / #46192b / #6A394C). No new hues.
Sections are assigned different existing shades so the page has depth and
adjacent sections don't merge into each other.

Accessibility and motion

• Decorative SVGs are aria-hidden and pointer-events-none
• Animations respect prefers-reduced-motion

One copy deviation from #146

In the closing section, "Join our newsletter and the Discord to stay up to date
with Payjoin development" became two CTA buttons plus a caption. Same links,
same destinations — keeping the sentence and adding buttons would have meant
four links to two places. Happy to revert if you'd rather the copy stay
untouched.

Notes

• yarn.lock deliberately excluded — no dependency changes
• The 6 pre-existing Cannot find namespace 'JSX' errors from yarn typecheck
  are untouched by this PR

[aizuanjeme]

Hey @DanGould , just following up on this PR whenever you get a chance. Would appreciate a review when you have some time. Thanks!