Dropdown: replace suppressed-axe panel demos with a proper Popover surface (a11y)

[lifeiscontent]

The Dropdown 'Display Properties' / 'Display Accordion' demos render radios/checkboxes/pills inside a role="menu" surface, which isn't valid ARIA, so 3 axe rules are disabled to pass the gate:

• aria-allowed-attr (×2, dropdown.stories.tsx ~716/848)
• aria-required-children (×1, ~1004)

These are the only suppressed a11y rules in the whole library. Real fix: build a dedicated Popover (non-menu) surface for settings-panel-style content so those patterns are valid without disabling axe rules, then remove the suppressions. Related: #16 open questions.

[lifeiscontent]

Resolved in #53.

Added a dedicated non-menu Popover surface (@plane/propel/components/popover, Base UI Popover) and moved the DisplayProperties + DisplayAccordion panels onto it. On the non-menu surface the radios/checkboxes/pills are valid children, so the two aria-allowed-attr suppressions are removed and axe passes clean for both panels.

Note on the third rule: the aria-required-children suppression on the Submenu story (~L1004) is kept. That one is a genuine Base UI submenu markup pattern (a <span aria-owns> menuitem child of the parent role="menu"), unrelated to the panel issue and not fixable by a Popover, since a submenu is a real menu. I confirmed it by removing the suppression and watching the violation come back. Documented in the story comment.