App layout refactor: Phase 1

[RobertJoonas]

Intro

Kick off a gradual modernization of our app's layouts, moving away from the old/classic Phoenix .heex layouts towards function components. Rather than one large re-write, we'll end up in a state where:

1. Every existing template continues to work with a Layout.legacy component (i.e. current app.html.heex, but delegating to a new <.app> layout under the hood)
2. It's possible to completely opt out from that legacy layout and use <.app> layout directly, feeding it whatever assigns necessary, straight from the template context

Phases

Phase 1 [This PR] — Turn the root layout's body into a function component
Phase 2 — Introduce Layouts.app and use it in Layout.legacy
Phase 3 — Let individual pages opt out of legacy layout. Do so for auth pages.

This PR

• Introduce Layouts.legacy -- a function component that renders the whole body of the app, given the whole conn.assigns object (incl. @inner_layout, @inner_content)
• bg_class and background (color) assigns still need to stay in app.html.heex because otherwise dark/light flickers will surface

Tests

• This PR does not require tests

Changelog

• This PR does not make a user-facing change

Documentation

• This change does not need a documentation update

Dark mode

• This PR does not change the UI