fix(core): keep ActionPanel submenu open when hovering into it

[kinged007]

Fix: ActionPanel source submenu closes when moving the mouse into it

Fixes #16

Problem

In the element inspector's action panel, hovering a source entry opens the plugin submenu (code preview + "Add comment" / "Copy path" actions), but the submenu disappears as soon as the mouse moves into it — even while the pointer is still inside the popup. Clicking entries is also flaky ("buttons sometimes unclickable") because the menu can close before the click lands.

Root cause

The Submenu used DropdownMenu.Trigger with openOnHover, delay={0}, closeDelay={0}. Base UI's hover-close path is governed by floating-ui's safePolygon, which only keeps the popup open if the pointer travels through a narrow safe zone between the trigger and the popup:

• The popup (up to ~480px wide, e.g. the folder-access prompt) is much taller and wider than the ~315px trigger entry, and when the viewport lacks room it flips to the opposite side (side="right" with collisionPadding).
• The safe-polygon geometry is anchored once at the pointer-leave position and can be near-degenerate (the cursor point is offset by only POLYGON_BUFFER / 2 = 0.25px from the cursor), so any diagonal traversal or small vertical deviation while crossing the 4px gap exits the polygon and closes the menu — even though the pointer is about to enter (or is already inside) the popup's bounding box.
• closeDelay={0} means any missed "landing" closes the menu immediately.

Fix

Replace the openOnHover/safePolygon close behavior with a standard hover-intent pattern that react-trace fully controls:

• Controlled open state on the DropdownMenu.Root.
• Open on hover/focus of the trigger; close with a 150ms grace period (SUBMENU_CLOSE_DELAY_MS) that is cancelled whenever the pointer is over the trigger or the popup.
• Set the selected source (setSelectedSource) when opening via hover too — previously the hover-open path (which goes through the controlled open prop) never ran setSelectedSource, so the plugin action panels rendered an empty popup on the first hover until a click opened the menu through onOpenChange.

Verification

Reproduced the original bug headlessly against a dev server (popup closed 3px inside its edge while moving the mouse in), then verified the fix:

• 25-step approach into the popup: stays open through every step (was closing at step ~24).
• Fast single-jump move into the popup: stays open (was closing on the first move).
• Fresh page load, first hover: popup opens at full size with content, stays open when moving in.