Add "Why us" landing page to flutter.dev

[vmkstudio]

Note

Opening as a draft for early feedback on approach and on the open
questions at the bottom — particularly how to host the whitepaper PDF.

Adds /why-us, a marketing page making the business case for adopting
Flutter, implemented from a design handoff.

Approach

The goal was to reuse what sites/www already has rather than introduce a
parallel set of conventions:

Concern	Reused
Header and footer	DefaultLayout — the page adds neither
Closing call to action	The shared CTASection, unchanged
Icons	The existing Icon component (Material Symbols Rounded)
Buttons	.btn from components/_btn.scss
Colour, radius, spacing	Tokens from core/_vars.scss
Breakpoints	breakpoints.screen() mixins, mobile-first

Nine of the design's eleven type sizes map exactly onto the existing heading
scale. The section subhead moved 32px → 36px (--font-size-heading-2) and the
case quote landed on 36px exactly. Only the stat numerals (3X, 97%) and the
fixed hero stage geometry are literals, as no token covers them.

New files

• content/why-us/index.md — page content and frontmatter
• lib/src/pages/why_us_page.dart — composes the sections
• lib/src/components/sections/why_us_*.dart — six page sections
• lib/src/utils/visibility_observer.dart — scroll-into-view trigger and tween
• lib/styles/pages/_why_us.scss — page styles

Note for reviewers

The four sections that hydrate on the client take their asset URLs as
constructor parameters instead of calling context.asset() themselves.
utils/asset_utils.dart imports dart:io, so calling it from a @client
component pulls the server-only SDK into the client bundle and
build_web_compilers silently skips compiling main.client.dart.js —
disabling interactivity across the whole site, not just this page. Worth
knowing about if other pages later add client-side sections.

Verification

• dart analyze — clean
• dart run dash_site --site=www build — succeeds; /why-us generated
• Client bundle compiles (322,812 bytes vs 318,796 on main), no skipped
  entrypoints
• All 28 assets referenced by the page resolve in the build output

Open questions

1. The whitepaper PDF is 7.1 MB and committed to the repo. Every other PDF
   the site links to is hosted externally. Happy to swap the link for a CDN URL
   if you'd prefer to host it there — probably the right call.
2. The "One Design System" card uses the hub symbol where the design had a
   bespoke SVG. Close, but not identical.
3. The page is not yet linked from the header nav; let me know where it belongs.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.