Drag calendar events to another day in the week grid (#400)

A timed event in the week view now moves sideways across the columns
as well as up and down, landing on the new day at the hour it was
dragged to. All-day chips above the grid drag between days the way
month chips do. Both drags count from the day the event was picked up
on, so a multi-day event grabbed on its last day moves by the distance
dragged, not by its length.

web/src/lib/calendar/__tests__/eventDrag.test.ts

@@ -10,6 +10,8 @@ import {
   snap,
   movePatch,
   moveByDaysPatch,
+  moveAcrossPatch,
+  columnsMoved,
   dayDelta,
   resizePatch,
   SNAP_MINUTES,
@@ -180,10 +182,22 @@ describe("the patch a drag sends, computed in the event's own frame", () => {
     expect(resizePatch(3600, -600)).toEqual({ duration: "PT15M" });
   });
 
+  it("moves by days and minutes together, as a week-grid drag does", () => {
+    expect(moveAcrossPatch("2026-09-04T14:00:00", 2, 90)).toEqual({ start: "2026-09-06T15:30:00" });
+    expect(moveAcrossPatch("2026-09-04T14:00:00", -1, 0)).toEqual({ start: "2026-09-03T14:00:00" });
+    expect(moveAcrossPatch("2026-09-04T14:00:00", 0, -30)).toEqual({ start: "2026-09-04T13:30:00" });
+  });
+
+  it("adds the days as days, so a clock change does not move the hour", () => {
+    // US clocks go back on 1 November 2026; 14:00 stays 14:00 across it.
+    expect(moveAcrossPatch("2026-10-31T14:00:00", 2, 0)).toEqual({ start: "2026-11-02T14:00:00" });
+  });
+
   it("says nothing at all about a start it cannot read", () => {
     expect(movePatch("not a date", 30)).toEqual({});
     expect(moveByDaysPatch("", 3)).toEqual({});
     expect(moveByDaysPatch("2026-09-04T14:00:00", Number.NaN)).toEqual({});
+    expect(moveAcrossPatch("not a date", 1, 30)).toEqual({});
   });
 });
 
@@ -228,3 +242,23 @@ describe("pixelsToMinutes", () => {
     expect(SNAP_MINUTES).toBe(15);
   });
 });
+
+describe("columnsMoved", () => {
+  it("counts whole columns, to the nearest", () => {
+    expect(columnsMoved(100, 100, 2, 7)).toBe(1);
+    expect(columnsMoved(140, 100, 2, 7)).toBe(1);
+    expect(columnsMoved(160, 100, 2, 7)).toBe(2);
+    expect(columnsMoved(-40, 100, 2, 7)).toBe(0);
+    expect(columnsMoved(-160, 100, 3, 7)).toBe(-2);
+  });
+
+  it("stops at the edges of the week instead of wrapping", () => {
+    expect(columnsMoved(-900, 100, 2, 7)).toBe(-2);
+    expect(columnsMoved(900, 100, 2, 7)).toBe(4);
+  });
+
+  it("never moves sideways in a one-day grid or before it is measured", () => {
+    expect(columnsMoved(500, 100, 0, 1)).toBe(0);
+    expect(columnsMoved(500, 0, 0, 7)).toBe(0);
+  });
+});