fix(web): align dispatch popover with trigger right edge

[bobleer]

Summary

• Switch dispatch target picker from createPortal + position: fixed to inline position: absolute with right: 0; bottom: calc(100% + 7px), matching the permission-mode popover pattern
• Replace overflow: auto with overflow-x: hidden; overflow-y: auto to prevent horizontal scrollbar on hover
• Remove JS-based position computation (computeFixedPopoverPosition, resize/scroll listeners) — CSS-only positioning eliminates the initial-render flash that caused the menu to occasionally appear outside the app window

Test plan

• Click the dispatch target button — popover should open above, right-aligned with the button's right edge
• Hover over menu items — no horizontal scrollbar should appear
• Resize the window while the popover is open — it should stay correctly positioned
• Open the popover multiple times rapidly — it should never flash at (0, 0) or appear outside the app
• Verify the permission-mode popover still works correctly (unchanged)