Fix cursor overlay invisible on secondary monitors

[Catafal]

Feature

Fix multi-monitor cursor overlay rendering (resolves #24)

Summary

• The blue cursor companion was invisible on any monitor that is not the primary display
• Root cause: NSHostingView.frame uses window-local coordinates (always origin 0,0), but was being assigned screen.frame which has a non-zero origin for secondary monitors (e.g. {x:1920, y:0})
• For a secondary monitor at x=1920, this placed the hosting view 1920pt to the right inside the window — completely off-screen

Changes

leanring-buddy/OverlayWindow.swift — one line in OverlayWindowManager.showOverlay():

// Before (broken for secondary monitors):
hostingView.frame = screen.frame

// After (correct — window-local coordinates):
hostingView.frame = CGRect(origin: .zero, size: screen.frame.size)

The NSPanel window itself is already positioned correctly at screen.frame.origin via OverlayWindow.init — only the content view frame needed fixing.

Why only the primary monitor worked before

Primary monitor has screen.frame.origin = (0,0), so screen.frame accidentally produced the correct window-local rect. Every secondary monitor has a non-zero origin and was broken.

Testing

• GIVEN two monitors connected, WHEN ctrl+option pressed with mouse on secondary display, THEN cursor appears on that display ✓
• GIVEN Claude responds with [POINT:x,y:label:screen2], THEN cursor animates to correct position on secondary display ✓
• GIVEN only primary monitor, THEN behavior unchanged (origin 0,0 produces identical rect) ✓

Review Notes

All downstream coordinate math (convertScreenPointToSwiftUICoordinates, isCursorOnThisScreen, startNavigatingToElement) was already correct and required no changes. Verified by tracing the full coordinate pipeline across 3 independent passes.

Closes #24