Phone: toast covers the selection sheet's last row; 'Move to trash' cannot be tapped #405

Where: web/src/styles/app.css:894-909 (.toast is fixed at bottom: var(--space-3), z-index 9) and the fixed .selection-bar sheet at the bottom (app.css:679-685, z-index 3).

Steps (Pixel 7, en):

1. In a folder, open Upload ⌄ → New folder, type "Sub" and press Enter.
2. The new folder is selected, so the selection sheet shows, and the toast "Created folder Sub" appears at 761-827 px.
3. The sheet's last button, "Move to trash", sits at 783-827 px. elementFromPoint at its centre hits the toast, so it cannot be tapped.

A success toast hides the button for 5 s. An error toast (such as "0 of 1 copied") stays until it is closed (DESIGN.md, "Toast"), so the destructive action stays hidden that long. The upload toast covers the last list row in the same way.

Expected: On phone width the toast sits above the selection sheet, and every action in the sheet stays reachable.

Fix: On phone width, set the toast to bottom: calc(var(--selection-bar-height, 0px) + var(--space-3)). FolderPane already measures the sheet's height into --selection-bar-height (FolderPane.vue:730); set that variable on an ancestor the toast also sits under, such as <html> or the app shell, while a selection exists.

Test (Playwright, mobile project): Create a folder through the Upload menu (it gets selected and the toast shows). Assert that the toast's bounding box does not intersect the box of .selection-bar, and that a trial click on "Move to trash" succeeds. Fails today.

Found by the automatic review round (claude-opus-5-5, b739414); per CLAUDE.md the fix starts with a failing behaviour test.