fix(panel): clamp panel window height to screen (#304)

[LikeSundayLikeRain]

Fixes #304.

Symptom

The Ask-user-question / permission panel becomes invisible ~2s after it appears, on some displays. Moving the mouse over the panel area brings it back; moving away hides it again. Reproduces on the built-in display and one external monitor, but not another external display, across two different MacBook Pros.

Root cause

The island is a single borderless, non-opaque NSPanel whose height is sized up-front to fit the largest possible expanded session list:

let maxH = max(300, maxSessions * 90 + 60)   // maxVisibleSessions * 90 + 60

With a large maxVisibleSessions (e.g. 99) this makes the window 8970pt tall — a ~18k-pixel-tall backing store on a Retina display, for a window that is ~95% transparent and only ever draws content in its top ~200pt.

On macOS 26, the WindowServer drops compositing for a backing store that large once the surface goes idle (to reclaim GPU memory). Nothing in the app forces a repaint of the settled card, so it renders blank until a mouse event forces a recomposition — hence "invisible after ~2s, reappears on hover." It is display-dependent because a 1× (non-HiDPI) external monitor's smaller buffer stays under the purge threshold, which is why one external display was unaffected.

Fix

Clamp the panel window height to the screen's visible height. The content is top-anchored and the session list already scrolls internally (ThinScrollView caps at maxVisibleSessions * 90), so the window never needs to be taller than the screen.

Verification

Confirmed on macOS 26.6 with a release build, using the real hook path (a genuine AskUserQuestion over the hook socket), on a Retina display:

• Instrumented the live window: while the card was blank it still reported isVisible=true, alpha=1.0, occlusionState.contains(.visible)=true, surface .questionCard — ruling out collapse / orderOut / alpha / occlusion-state. The frame was 8970pt tall.
• Reducing the window height (via maxVisibleSessions) made the card stay visible with nothing else changed.
• With this clamp and maxVisibleSessions=99, the card now stays visible for the full duration (previously it vanished after ~2s).