v2.5.3 - Phone layouts, menus and chat fixes

Patch release. Two changes may need a look in your code:

• spacing on MStack is now the gap between children. Before, it added a margin on every side. If you relied on that margin, use padding (inside) or my / mx (outside).
• Framework styles sit in the mineral cascade layer, so your CSS always overrides components. Global element rules such as a { color } or button { } now win over components too, and :root theme tokens in your stylesheet now take effect.

Also in this release:

• MHeading: new align prop and tone="inherit"; heading sizes come from the --mineral-font-size-h1…h6 tokens and h1–h3 shrink on phones. A coloured MText as="span" inside a heading keeps the heading font and line height.
• MText: tone="inherit" takes the parent colour.
• MNavbar: the mobile menu is opaque (token --mineral-navbar-menu-bg) and the drawer is positioned against the window.
• MGitHubIcon works with color="inherit".
• MChat (Pro): line breaks are kept, image-only messages have no empty bubble, groups show their title initials or the new avatar, MChatHeader takes actions, new showSender.