Welcome modal on logged-out homepage (#8944)

* Adds welcome modal to logged-out homepage

* Adds metrics and feature gate for welcome modal

* Slightly smaller text for mobile screens to avoid wrapping

* Remove unused SVG

* Adds text gradient and "X" close button

* Fix color on "Already have an account?" text

* tweak hooks, react import

* rm stylesheet

* use hardcoded colors

* add focus guards and scope

* no such thing as /home

* reduce spacign

* use css animations

* use session storage

* fix animation fill mode

* add a11y props

* Fix link/button color mismatch, reduce gap between buttons, show modal until user dismisses it

* Fix "Already have an account?" line left-aligning in small window sizes

* Adds "dismissed" and "presented" metric events

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Co-authored-by: Samuel Newman <mozzius@protonmail.com>

src/alf/atoms.ts

@@ -1063,6 +1063,7 @@ export const atoms = {
   }),
   fade_out: web({
     animation: 'fadeOut ease-out 0.15s',
+    animationFillMode: 'forwards',
   }),
   zoom_in: web({
     animation: 'zoomIn ease-out 0.1s',