App layout refactor: Phase 2

[RobertJoonas]

Changes

See phase 1 PR for full context.

• Add a new Layouts.app component: typed attrs (header?, footer?, global_notices?,
  trial_badge?, embedded?, load_dashboard_js?), all defaulting to today's
  shown-by-default behavior. Layouts.legacy becomes a thin adapter translating the
  old hide_header?/hide_footer?/disable_global_notices?/hide_trial_badge?
  assigns into the new attrs — no existing call sites change.

• _header/_footer/_notice/_flash partials converted to real function
  components (components/layouts/*.html.heex, via embed_templates), dropping
  @conn in favor of plain assigns and PlausibleWeb.Endpoint for route helpers.
  Renamed the billing-notices partial to team_notices to avoid colliding with
  Generic.notice/1.
• LayoutView.home_dest/1 now takes current_user directly instead of conn.

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