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/components/hooks/useWelcomeModal.native.ts

@@ -0,0 +1,3 @@
+export function useWelcomeModal() {
+  throw new Error('useWelcomeModal is web only')
+}