fix: position browser context menu at cursor with edge flipping

[Jinwoo-H]

Summary

• Use screen.getCursorScreenPoint() in main process as ground truth for cursor position, immune to guest/renderer coordinate space mismatches
• Convert screen coordinates to CSS viewport via (screenX - window.screenX) / zoomFactor in the renderer, handling UI zoom correctly
• Flip the context menu to the opposite side of the cursor when it would overflow viewport edges (right, bottom, or corner)
• Self-correct via useLayoutEffect + getBoundingClientRect() to compensate for any CSS containing block offsets
• Dismiss context menu on left/middle click in guest webview but skip right-click to avoid flash from dismiss-then-reopen

Test plan

• Unit tests for screen-to-viewport coordinate conversion at multiple zoom levels
• Unit tests for CSS containing block self-correction math
• Unit tests for viewport edge flipping (right, bottom, corner, clamp-to-zero, boundary)
• Unit tests for context menu IPC dispatch (params passthrough, navigation state, renderer unavailable)
• Unit tests for dismiss behavior (left-click, middle-click, right-click skip, mouseMove ignore)
• Unit tests for cleanup/teardown
• Manual: right-click in browser pane → menu top-left at cursor
• Manual: right-click near right/bottom edge → menu flips direction
• Manual: right-click at various UI zoom levels → menu stays at cursor