feat: support moving taskbar widget between monitors

[k7631159]

Summary

• detect primary and secondary Windows taskbar windows
• persist the selected taskbar index so the widget reopens on the last-used monitor
• allow dragging the taskbar widget across monitors by dropping it onto another taskbar
• clamp saved taskbar offsets so the widget stays visible when moving between taskbars with different widths
• make the whole widget draggable instead of only the left divider area

Draft status

This is ready for early feedback, but I am opening it as a draft because I would like to do one more manual pass on my own multi-monitor setup before marking it ready for review.

Notes

This keeps the existing taskbar-widget model rather than adding a floating window mode. Users can move the widget between taskbars by dragging it directly to another monitor's taskbar. There is no menu command for switching taskbars, because carrying a large-monitor offset back to a smaller taskbar can place the widget off-screen.

Validation

• rustfmt --check src/native_interop.rs src/window.rs
• cargo build --release
• cargo clippy passes with existing upstream warnings
• manually verified the widget stays visible when a previously-large tray_offset is clamped on a smaller taskbar
• manually verified dragging from the body of the widget updates its taskbar offset
• verified the cross-taskbar drop handler moves the widget from the primary taskbar to a secondary taskbar

Follow-up / compatibility

There is an open upstream PR for Explorer restart recovery (#36). If that lands first, this branch may need a small rebase around taskbar handle discovery and recovery logic.