App layout refactor: Phase 3

[RobertJoonas]

Changes

See phase 1 PR for full context.

Review recommendations:

1. Review commit by commit -- I've split each template migration into a separate commit.
2. Hide whitespace -- whole templates wrapped in Layouts.auth

• Add a legacy_layout? assign (default true) on the shared root layout. When
  false, the root renders a plain @inner_content instead of going through
  Layouts.legacy — the mechanism for a page to fully own its own layout.

• New Layouts.auth wraps its content in
  <.app header?={false} footer?={false} global_notices?={false}> directly,
  providing the centered logo/heading/subtitle that the new auth pages need.
• Removed AuthLayoutContext and ControllerHelpers.render_auth_page — call sites
  just pass legacy_layout?: false alongside their existing assigns.
• All auth templates wrapped in <Layouts.auth>; deleted the old
  templates/layout/auth.html.heex secondary layout.

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