fix: let the webcam preview bubble cross monitor boundaries while dragging

[Alfredoalv13]

Summary

• The floating camera preview bubble had the same single-display limitation the HUD control bar had before fix: let the HUD widget cross monitor boundaries while dragging #4 — drag math clamped to window.innerWidth/innerHeight (one display's work area), so it could never be dragged onto a second monitor.
• Applies the same fix pattern as fix: let the HUD widget cross monitor boundaries while dragging #4: track a fixed grab offset (distance from the bubble's edge to the click point) instead of a client-coordinate delta snapshot, compute the target position in absolute screen coordinates (event.screenX/screenY minus window.screenX/screenY) so it stays correct even if the HUD window relocates mid-drag, and re-measure the bubble's actual rendered rect every frame so any mid-drag window resize is absorbed automatically instead of causing a jump.
• Reuses the existing hudOverlayRelocateToPoint IPC call — the webcam bubble renders inside the same HUD overlay window as the control bar, so no main-process changes are needed here.

Why

Confirmed by reading the code: PR #4 only touched useHudBarDrag.ts (the control bar). This hook (useWebcamPreviewOverlay.ts) has its own, separate drag implementation with the identical bug and no relocate mechanism at all.

Test plan

• npx tsc — clean
• npx vitest --run — 786/786 passing
• npx biome check on the changed file — clean (one pre-existing, unrelated formatting issue on main left untouched)
• Manual: drag the floating webcam preview bubble from the laptop screen onto a second monitor, confirm it crosses cleanly with no jump

[Alfredoalv13]

Internal bug-hunter review

Reviewed against the reference implementation (useHudBarDrag.ts, merged in #4) with focus on what's different in this port: the drag-threshold gating, the unconditional pointerUp recompute, external consumers of the ref's shape, and the two independent relocate in-flight guards.

No confirmed bugs. Summary:

• Screen-coordinate threshold check is sound (actually more robust than the old client-coordinate diff, since it's immune to spurious deltas from external window movement).
• The unconditional pointerUp position recompute (matching the HUD bar's pattern) can nudge the bubble by at most the 6px drag threshold on a plain click — negligible, and nothing reads the bubble's exact position from a click handler.
• Confirmed via grep: nothing outside this hook depends on the old drag-state ref shape (startX/initialLeft/etc.) — only truthiness is read externally.
• The two hooks' independent relocate in-flight guards are safe: the main-process handler is stateless and idempotent per call, so there's no read-modify-write race between them.

One pre-existing, out-of-scope note: the unmount cleanup effect doesn't null webcamPreviewDragStartRef.current (unlike the reference's cleanup, which does null its equivalent) — but that line wasn't touched by this diff, so it's not this PR's concern.

Ready for your manual verification (drag the bubble across a real second monitor) before merge.