fix(menubar): make provider strip reachable and mouse-wheel scrollable

[vaibhavarora14]

Summary

• Add overflow-aware left/right chevron nav buttons to AgentTabStrip so off-screen providers are reachable when chips don't all fit in the popover.
• Translate vertical-only mouse-wheel scroll events into horizontal deltas while the cursor is over the strip so standard mice can scroll it.

Why

The strip already lays out provider chips inside a horizontal ScrollView, which on macOS:

1. Does not scroll from click-drag, so when chips overflow the viewport at the default 360pt popover width (~7+ providers), the off-screen chips are completely unreachable.
2. Ignores vertical-only NSEvents. Standard mouse wheels emit hasPreciseScrollingDeltas == false with a non-zero deltaY and zero deltaX, so a horizontal ScrollView silently drops the event. Trackpads, which produce precise deltas with native horizontal components, already worked.

Either bug alone is annoying; together they make some providers unselectable on a mouse-only setup.

Implementation notes

• Wrap the strip in ScrollViewReader. Add left/right chevron buttons that programmatically scroll to the next/previous visible provider via proxy.scrollTo(target.id, anchor: .center) and switch the selected provider via the existing store.switchTo(provider:). Buttons are only rendered when stripContentWidth > stripViewportWidth - 30, and they disable at the strip's ends.
• Install an NSEvent.addLocalMonitorForEvents(matching: .scrollWheel) in .onAppear (removed in .onDisappear). Track strip hover via .onHover into a @MainActor singleton AgentTabStripScrollState so the monitor closure can read the latest hover state without capturing stale SwiftUI @State.
• In the monitor, gate the rewrite on:
  • cursor is currently over the strip
  • !event.hasPreciseScrollingDeltas (mouse wheel, not trackpad)
  • |deltaX| < 0.001 and |deltaY| > 0 (vertical-only delta)
    When all hold, copy the CGEvent and transpose scrollWheelEventDeltaAxis1 / PointDeltaAxis1 / FixedPtDeltaAxis1 onto axis 2, then return the rewritten event so the underlying NSScrollView receives a real horizontal delta. All other events (trackpad, vertical scroll outside the strip, etc.) are passed through untouched, so vertical scrolling of the main popover content is unaffected.

Test plan

• Run the menubar app with 7+ detected providers at the default popover width. The provider strip shows left/right chevrons.
• Click the chevrons. Hidden providers become visible and selectable; chevrons disable at the strip ends.
• With a standard mouse wheel, hover the strip and scroll. The strip scrolls horizontally.
• With a trackpad, two-finger horizontal swipe over the strip still scrolls it.
• With a standard mouse wheel, hover the main popover content (outside the strip) and scroll. Vertical scrolling of the main content still works as before.
• Open the popover on a configuration where providers fit without overflow. Chevrons do not appear.

Fixes #333. Related to #332.

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