feat: modernize Menu to the latest Material Design specs

[adrcotfas]

• Modernize the Menu component to the latest specs. Reuse the newly introduced theme tokens (typography, motion, shape etc).
  See https://m3.material.io/components/menus for the full picture.
• Extract component specific tokens for easier maintenance.
• Get inspired from TextInput/TextField, Switch, Checkbox, FAB components which have gone through modernization.

Current State

Menu

MD3 spec: Menus
Specs link: https://m3.material.io/components/menus/specs

Color tokens

• Container background: theme.colors.elevation[level2] which resolves to surfaceContainerLow ✅ (standard color scheme)
• Menu.Item label text: onSurface ✅
• Menu.Item leading/trailing icon: onSurfaceVariant ✅
• Menu.Item disabled opacity: stateOpacity.disabled (0.38) ✅
• No selected item state: spec defines tertiaryContainer bg / onTertiaryContainer text+icon for selected menu items
• No vibrant color scheme (M3 Expressive): tertiaryContainer container / onTertiaryContainer icons+labels / tertiary selected bg / onTertiary selected text+icon

Spacing / sizing

• Container paddingVertical: 8 ✅
• Item min/max width: 112dp / 280dp ✅
• Item paddingHorizontal: 12 ✅
• Icon leading margin: 12dp after icon ✅
• Item height: 48dp (standard), 32dp (dense) – baseline menu values ✅
• Menu.Item label text uses bodyLarge; spec requires labelLarge
• Menu.Item has no supporting text slot (bodySmall); spec anatomy includes optional supporting text per item
• Menu.Item has no trailing supporting text slot (labelLarge); spec anatomy includes optional trailing text per item

Shape

• Container: borderRadius: theme.roundness = 4dp; spec corner.large = 4 * roundness = 16dp ❌
• First child item: spec corner.medium top corners; not implemented
• Last child item: spec corner.medium bottom corners; not implemented
• Selected item: spec corner.medium; not implemented
• No per-item shape rounding at all; all items are rectangles

Behavior

• Open/close: scale + opacity animation ✅ (but uses Easing.bezier(0.4, 0, 0.2, 1) from old M3 motion guidelines; M3 Expressive uses spring-based motion)
• Dismisses on outside press ✅
• Keyboard-aware positioning ✅
• RTL support ✅
• Escape key dismissal on web ✅
• Back handler on Android ✅
• No item selection state / active item support
• No grouped layout (gaps between item groups); current workaround uses Divider which is not the spec-aligned grouping mechanism for vertical menus
• No submenu support
• No badge support on items
• M3 Expressive vertical menu shape morphing (active container shape animates as focus moves to submenu) not implemented

Notes

• The current component implements the baseline menu variant only. The M3 Expressive vertical menu is a largely new feature surface (new shapes, selection states, vibrant color scheme, submenu motion, grouped layout) that requires significant additions.
• The borderRadius: theme.roundness bug is a significant visual regression vs spec – the container should be visibly rounded.