feat: offer undo at the moment something is removed

Undo existed but nothing said so. The only route was Cmd/Ctrl+Z, which was
documented in a README nobody reads mid-task, and which does not exist on
a phone at all. On touch there was simply no way to get a deleted task
back.

Deleting a task or clearing completed ones now brings up a short-lived bar
with a real Undo button, so touch has a route for the first time. It also
shows the keyboard shortcut, which is the point: someone learns it once,
here, at the only moment they actually care.

It uses the existing tokens, sits out of the way at the bottom, clears
itself after seven seconds and goes as soon as it is used. The shortcut
badge is hidden where no keyboard exists rather than advertising a chord
nobody can press.

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** - `Cmd/Ctrl+Z` brings back a deleted task, in its original position
+- ↩️ **Undo** - Deleted 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**: Press `Cmd/Ctrl+Z`. The task returns to where it was
+- **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
 - **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