Remove inner border from popup menus

[devin-ai-integration]

Summary

Removes the border Tailwind class from the default variant of all popup container components in the shared UI package, per design feedback to reduce visual noise. The shadow (shadow-md/shadow-lg) is retained for depth.

Only the default variant is affected — the app variant (which uses appFloatingContentClassName) is unchanged.

Affected components:

• DropdownMenuContent and DropdownMenuSubContent
• ContextMenuContent and ContextMenuSubContent
• PopoverContent

Review & Testing Checklist for Human

• Visually verify the overflow menu (three-dot "..." button on sessions) looks correct without the border
• Check that PopoverContent consumers outside of menus (e.g., comboboxes in settings, participant dropdowns) still look good without the border — this change is global to all default-variant popovers
• Right-click context menus should also be checked for visual consistency

Notes

• This is a CSS-only change (removing border from Tailwind class strings in the default variant). No logic changes.
• The app variant is not affected since it uses appFloatingContentClassName directly.
• Could not visually test since the Tauri desktop app cannot run in this environment. Manual visual QA is required.

Link to Devin session: https://app.devin.ai/sessions/8cc78905d7284bf6892f2aae4a80204a
Requested by: @ComputelessComputer

[devin-ai-integration]

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