Make context menus keyboard and screen reader operable

[captbaritone]

Webamp's click-to-open menus can't be opened, navigated or closed from the keyboard, and screen readers don't see them as menus. These are the main window's Winamp Menu (#option), the clutter bar's Options menu (O), the equalizer's Presets menu, and the playlist's Sort and Misc Options menus.

Menu button (ContextMenuTarget)

• The trigger (its .handle) is now a focusable role="button" with aria-haspopup="menu" and aria-expanded.
• It takes a new required label prop for its accessible name: "Winamp Menu" (matching #option's existing title), "Options", "Presets", "Sort List" and "Misc Options".
• Enter, Space or ArrowDown opens the menu and focuses its first item, and Escape closes it.
• Opening with the mouse is unchanged, and doesn't move focus into the menu.

Menu (ContextMenu)

Roles:

• the list is role="menu"
• items are menuitem, or menuitemcheckbox with aria-checked when they have a checked prop
• <Hr> is a separator
• <Parent> is a menuitem with aria-haspopup/aria-expanded and a nested menu
• items get aria-label, so a parent's name doesn't include all of its children's text, and a checked item's name doesn't include the CSS "✓"

Keyboard (roving focus, items are tabIndex={-1}):

Key	Action
↓ / ↑	Next / previous item, wrapping and skipping separators
Home / End	First / last item
Enter / Space	Activate the item, close the menu, return focus to the trigger
→ (or Enter/Space on a parent)	Open the submenu and focus its first item
← / Escape inside a submenu	Close the submenu and focus its parent
Escape / Tab	Close the menu and return focus to the trigger

• Handled keys call stopPropagation(), so with enableHotkeys on, the arrow keys don't also change volume or seek.
• Submenus still open on hover via CSS. Keyboard-opened ones get an .expanded class.

Visuals: keyboard focus uses the existing hover highlight via :focus-visible. The menu is portalled outside #webamp, so this PR also suppresses the browser's own focus outline there. Mouse users see no change.

Not in this PR

• Right-click menus (ContextMenuWrapper) get the same roles, and Escape closes them, but there's still no keyboard way to open them. Everything in them is also reachable from the Winamp Menu or as a button.
• The playlist window's sprite popups (Add/Remove/Select/Misc/List) are separate components and remain mouse-only.

Testing

• New ContextMenuTarget.test.tsx (12 tests) covers:
  • trigger semantics
  • opening with Enter, Space or ArrowDown (focus moves into the menu) vs the mouse (focus doesn't move)
  • item roles and state
  • arrow, Home and End navigation that skips separators
  • Enter activating an item and returning focus
  • Escape returning focus
  • opening/closing submenus, Escape inside a submenu, activating submenu items
  • navigation keys not reaching document-level hotkey listeners
• tsc, eslint, prettier --check and vitest run (220 passed) pass locally.

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[github-actions]

Size Change: +1.21 kB (+0.25%)

Total Size: 492 kB

📦 View Changed

Filename	Size	Change
packages/webamp/built/webamp.bundle.min.js	299 kB	+599 B (+0.2%)
packages/webamp/built/webamp.lazy-bundle.min.js	193 kB	+614 B (+0.32%)

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