Add a live-resize handle for the Paste Bar

[alvst]

The bar's height is only meaningful on screen, but the only way to change it was a Settings slider whose feedback was a number — and clicking into Settings dismisses the bar. This adds an optional drag handle on the bar itself, and makes the slider preview the size it's describing.

What changed

• BarResizeGeometry centralizes the height rules (default/min/max, per-display clamping, macOS 26 glass content-frame math) so the persisted preference and the live drag can't disagree.
• BarResizeHandle is a compact drag target at the top of the bar, with accessibility increment/decrement. A new "Show resize handle on the Paste Bar" setting gates it, off by default.
• The bar's key monitor steps aside while the handle holds first responder, so Escape and arrows during a drag reach the handle.
• Dragging the Settings height slider live-resizes the real bar when it's up. When it isn't, a translucent click-through ghost appears on the display under the pointer — a faded BarView behind a dashed outline with a large "N px" readout — and retires ~1.1s after the last change. Showing the bar clears any lingering outline.
• The slider's unit label reads "px" instead of "pt".

Screenshots

Before:

After:

The resize ghost is deliberately not pictured. It's a click-through window
that retires ~1.1s after the last change, so it can't be selected by macOS's
window capture and won't survive a screenshot — describing it is more honest
than shipping a still of the ordinary bar and calling it the ghost.

Notes for review

• The ghost is sized through this branch's own BarResizeGeometry, so the outline can't promise a height the display won't actually grant.
• The handle is off by default; with the setting off this is a no-op on the bar's appearance.
• No dependencies; merges in any order.
• Out of scope: no width or position adjustment, only height.

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This feature should be bundled into v2.

Part of #80.