fix: say what carry-over did, and stop the day control being a riddle

Two pieces of copy that were making the app harder to read than it needs
to be.

Carry-over happened in total silence. Open the app after a weekend and
unfamiliar tasks were sitting in Today with nothing explaining where they
came from, which reads as a bug rather than the feature it is. A line now
says how many were brought forward, on the day it happened, and it can be
dismissed.

The day control was a single button labelled with the day you were already
on, which then took you somewhere else. Clicking it was the only way to
find out what it did, and the date underneath already said where you were.
It is two options now with the current one marked, so the control states
both where you are and what the alternative is without being touched.

Also drops labelFor from dates.js along with its tests. It existed to
label that one button and nothing uses it now.

src/App.svelte

@@ -5,7 +5,7 @@
   import { cubicOut } from 'svelte/easing';
   import './style.css';
 
-  import { toKey, addDays, fromKey, labelFor, formatLong } from './lib/dates.js';
+  import { toKey, addDays, fromKey, formatLong } from './lib/dates.js';
   import { createStorage } from './lib/storage.js';
   import { rollover, msUntilNextMidnight } from './lib/rollover.js';
   import * as taskOps from './lib/tasks.js';
@@ -24,7 +24,19 @@
   storage.migrateLegacyKeys();
   const bootNow = new Date();
 
+  const bootTodayKey = toKey(bootNow);
+  const beforeRollover = storage.loadTasks(bootTodayKey).length;
+
   let tasks = $state(rollover(storage, bootNow));
+
+  /** How many unfinished tasks were pulled forward from earlier days on this
+   *  load. The app's cleverest behaviour used to happen in complete silence,
+   *  so returning after a weekend looked like a bug rather than a feature. */
+  let carriedOver = $state(Math.max(0, tasks.length - beforeRollover));
+
+  function dismissCarriedOver() {
+    carriedOver = 0;
+  }
   let newTask = $state('');
   // Seeded from the class the pre-paint script in index.html already set, so
   // there is one source of truth and no post-mount correction to flash.
@@ -474,9 +486,11 @@
     return toKey(addDays(fromKey(todayKey), 1));
   }
 
-  function switchDate() {
+  function showDay(key) {
+    if (key === selectedKey) return;
     cancelEdit();
-    selectedKey = selectedKey === todayKey ? tomorrowKey() : todayKey;
+    dismissUndo();
+    selectedKey = key;
     tasks = storage.loadTasks(selectedKey);
   }
 
@@ -494,7 +508,10 @@
     const wasViewingToday = selectedKey === todayKey;
 
     todayKey = toKey(now);
+    const before = storage.loadTasks(todayKey).length;
     const todayTasks = rollover(storage, now);
+    const moved = todayTasks.length - before;
+    if (moved > 0) carriedOver = moved;
 
     if (wasViewingToday) {
       selectedKey = todayKey;
@@ -521,7 +538,6 @@
   }
 
   const currentDateDisplay = $derived(formatLong(selectedKey));
-  const buttonText = $derived(labelFor(selectedKey, todayKey));
 
   const remainingTasks = $derived(tasks.filter(task => !task.completed).length);
   const completedTasks = $derived(tasks.filter(task => task.completed).length);
@@ -571,15 +587,26 @@
       </div>
       
       <div class="header-actions">
-        <button class="today-btn" aria-label="Switch between Today and Tomorrow" onclick={switchDate}>
-          <svg viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
-            <rect x="3" y="4" width="18" height="18" rx="2" ry="2" stroke="currentColor" stroke-width="2"/>
-            <line x1="16" y1="2" x2="16" y2="6" stroke="currentColor" stroke-width="2"/>
-            <line x1="8" y1="2" x2="8" y2="6" stroke="currentColor" stroke-width="2"/>
-            <line x1="3" y1="10" x2="21" y2="10" stroke="currentColor" stroke-width="2"/>
-          </svg>
-          {buttonText}
-        </button>
+        <!-- Both options are visible with the current one marked, so the
+             control no longer has to be clicked to find out what it does. -->
+        <div class="day-switch" role="group" aria-label="Choose a day">
+          <button
+            class="day-option"
+            class:selected={viewingToday}
+            aria-pressed={viewingToday}
+            onclick={() => showDay(todayKey)}
+          >
+            Today
+          </button>
+          <button
+            class="day-option"
+            class:selected={!viewingToday}
+            aria-pressed={!viewingToday}
+            onclick={() => showDay(tomorrowKey())}
+          >
+            Tomorrow
+          </button>
+        </div>
         
         <button 
           class="theme-toggle" 
@@ -634,6 +661,21 @@
         {/if}
       </div>
 
+      {#if carriedOver > 0 && viewingToday}
+        <div class="carried-notice">
+          <span>
+            {carriedOver === 1
+              ? 'One unfinished task carried over from an earlier day.'
+              : `${carriedOver} unfinished tasks carried over from earlier days.`}
+          </span>
+          <button class="carried-dismiss" onclick={dismissCarriedOver} aria-label="Dismiss">
+            <svg viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+              <path d="M18 6L6 18M6 6l12 12" stroke="currentColor" stroke-width="2" stroke-linecap="round"/>
+            </svg>
+          </button>
+        </div>
+      {/if}
+
       <div class="task-list">
         {#key selectedKey}
           {#if tasks.length === 0}