Smooth pane drags, overlays, and hover motion

[rorohecker]

Pane splitters used to ease on every drag frame, so the layout lagged the pointer and made scrolling/resizing feel drunk. This keeps tracking instant while dragging, and uses a short ease only for idle changes (double-click reset, preview open/close, simple-mode toolbar height).

Layout

• Splitter duration is 0ms during drag (layout_dragging), ~200ms cubic ease when idle
• Preview pane stays mounted while its width animates closed (preview_w > 0.5px + clip)
• Preview user-width is not double-eased; only preview_w animates

Spawn-in

• Overlays, dialogs, toasts, empty states, and the operations drawer fade and rise a few pixels into place instead of popping
• Command palette drops in from above
• Context menus use a shorter 90ms spawn so they still feel snappy
• Selection action bar fades in over the toolbar

Chrome

• Hover/press on buttons, tabs, chips, menus, toggles, and list headers: 90ms ease-out
• File rows stay instant on select/hover so virtualized list recycling does not ghost the previous file during scroll

viewport-y is intentionally not animated.

Verified with cargo clippy --lib --locked --offline -- -D warnings and cargo check --locked --offline. This is a native Windows app, so motion was not GUI-tested on this Linux VM.