fix: moving a task between days is no longer silent

Deferring was the one action that changed your list and said nothing about
it. The task disappeared, no message, and Cmd/Ctrl+Z would not bring it
back because the undo stack only recorded deletes and clear-completed.

That was worse than it sounds. The app had just taught people, via the bar
that appears when something is deleted, that things which vanish come with
a way back. Deferring quietly broke that promise, and anyone who misread
the arrow would reasonably believe they had destroyed the task.

It now uses the same bar, saying which way the task went, and undo sends
it back to the row it left rather than the bottom of the list. That needed
an insert position on moveTaskToDay, which is tested along with the round
trip, clamping and the default append.

Undo for a move is handled separately from applyUndo, which edits a single
day's array and cannot express something that spans two.

README.md

@@ -14,7 +14,7 @@ Built with Svelte for speed and simplicity. No overwhelming features, no endless
 
 ### Core Functionality
 - ✅ **Add, complete, and delete tasks** with smooth animations
-- ↩️ **Undo** - Deleted something by mistake? An Undo button appears, and `Cmd/Ctrl+Z` works too. The task returns to its original position
+- ↩️ **Undo** - Deleted or moved something by mistake? An Undo button appears, and `Cmd/Ctrl+Z` works too. The task returns to its original position
 - 📅 **Today & Tomorrow lists** - Plan ahead with separate task lists
 - 🔄 **Unfinished work carries over** - When a new day begins, tasks you didn't finish move to Today; completed ones are cleared away
 - ✏️ **Edit a task** - Fix a typo without deleting and retyping it
@@ -92,7 +92,7 @@ The date, storage, rollover, task and undo logic lives in `src/lib/` as plain mo
 - **Complete a task**: Click the checkbox next to the task
 - **Edit a task**: Click its text. Enter saves, Escape cancels, and clicking away saves too
 - **Delete a task**: Click the delete icon on the task (it shows on hover, and is always visible on touch), or press `Delete` with the task focused
-- **Undo a delete**: An Undo button appears for a few seconds after anything is removed. `Cmd/Ctrl+Z` does the same. Either way the task returns to where it was
+- **Undo**: An Undo button appears for a few seconds after a task is deleted, cleared or moved to another day. `Cmd/Ctrl+Z` does the same. Either way the task returns to where it was
 - **Move a task to Tomorrow**: Click the arrow on the task, or press `Alt+→`. From Tomorrow, the arrow points back and `Alt+←` returns it to Today
 - **Reorder tasks**: Drag with a mouse, long-press then drag on touch, or focus a task and press `Alt+↑`/`Alt+↓`
 - **Clear input**: Press Escape while the input is focused
@@ -130,7 +130,7 @@ Import only ever adds. Tasks already present are left alone, so importing the sa
 - **Escape**: Clear input field (when input is focused)
 - **Space/Enter**: Toggle task completion (when a task's checkbox is focused)
 - **Delete**: Delete the focused task
-- **Cmd/Ctrl+Z**: Undo the last delete or clear-completed
+- **Cmd/Ctrl+Z**: Undo the last delete, clear-completed, or move between days
 - **Alt+↑ / Alt+↓**: Move the focused task up or down
 - **Alt+→ / Alt+←**: Send the focused task to Tomorrow, or bring it back to Today