feat: add mt-divider component

[Arnold Stoba (arnoldstoba)]

What?

New mt-divider component: a horizontal or vertical separator line.

• orientation: horizontal (default) / vertical
• variant: solid (default) / dashed
• color: any border color token, defaults to color-border-secondary-default
• alignment: places slot content center (default), left, or right; the line on the aligned side is removed
• decorative: removes the separator semantics for purely visual lines
• full-bleed: stretches the divider beyond its container by the default card padding, e.g. to span the full width of a card
• optional slot for centered content (e.g. "or" in login flows, or a small action)

Closes #1197

Why?

Sections often need a stronger visual boundary than whitespace provides; until now there was no dedicated separator component (see #1197).

How?

• The line segments are ::before/::after pseudo-elements, so slot content renders centered between two flexible segments with a fixed gap; alignment suppresses the segment on the aligned side.
• Accessibility follows the separator role rules: role="separator" with matching aria-orientation by default, role="none" when decorative. With slot content the role is dropped entirely, because ARIA treats a separator's children as presentational — this keeps e.g. a button inside the divider reachable for assistive technologies.
• full-bleed applies negative margins of the default card padding (--scale-size-24): inline for horizontal dividers, block for vertical ones.

Testing?

• Unit tests for orientation, variant, color token, alignment, decorative/separator semantics, slot content accessibility, and the full-bleed class.
• Interaction test stories incl. nine visual test cases (default, dashed, brand color, with content, left/right alignment, vertical, full-bleed horizontal + vertical in a card).
• Full-bleed layout and alignment behavior verified in headless Chromium for both orientations.

Anything Else?

• Docs page with examples included (apps/docs).

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