fix(a11y): Phase 3 follow-up — implement ARIA radio-group keyboard pattern for energy selector

Codex post-implementation review flagged one MAJOR: the energy segmented
control declared `role="radiogroup"` / `role="radio"` but only wired
`onclick`. No arrow-key navigation, no Home/End, no roving tabindex.
Keyboard users got four independent tab stops while assistive tech was
told it was a single radio group — a broken ARIA contract.

Fix (W3C APG Radio Group pattern):
- Extract the options list as `energyOptions` so the render loop and
  the keyboard handler share one source of truth.
- `energyRadioKeydown` handles ArrowLeft/Right/Up/Down (cycle wraps),
  Home (first), End (last).
- Roving tabindex: the currently-checked button gets `tabindex=0`,
  the rest get `tabindex=-1`, matching the APG recipe. Focus moves
  with selection.
- The radiogroup container gets `tabindex="-1"` to satisfy the
  svelte-check a11y rule without creating its own tab stop.

All green: 251 tests, clippy -D warnings, fmt, svelte-check 0/0, build.

src/lib/pages/TasksPage.svelte

@@ -110,6 +110,56 @@
     }
   }
 
+  // ARIA radiogroup keyboard handling for the energy segmented control.
+  // Full W3C APG Radio Group pattern: arrow keys cycle, Home / End jump
+  // to ends, focus and selection move together. The options array lives
+  // here so the keyboard handler and the render loop share one source
+  // of truth — desynchronising them is the usual way these patterns
+  // rot over time.
+  type EnergyOption = { value: EnergyLevel | null; label: string };
+  const energyOptions: EnergyOption[] = [
+    { value: null, label: "—" },
+    { value: "high", label: "High" },
+    { value: "medium", label: "Med" },
+    { value: "brain_dead", label: "Low" },
+  ];
+  let energyRadioGroupEl = $state<HTMLDivElement | null>(null);
+
+  function selectEnergyByIndex(idx: number) {
+    const clamped = Math.max(0, Math.min(energyOptions.length - 1, idx));
+    const opt = energyOptions[clamped];
+    cycleCurrentEnergy(opt.value);
+    // Move focus to the newly-checked button so keyboard nav tracks
+    // selection, per the ARIA radio pattern.
+    const btn = energyRadioGroupEl?.querySelectorAll<HTMLButtonElement>('[role="radio"]')[clamped];
+    btn?.focus();
+  }
+
+  function energyRadioKeydown(e: KeyboardEvent) {
+    const current = energyOptions.findIndex((o) => o.value === settings.currentEnergy);
+    const here = current < 0 ? 0 : current;
+    switch (e.key) {
+      case "ArrowRight":
+      case "ArrowDown":
+        e.preventDefault();
+        selectEnergyByIndex((here + 1) % energyOptions.length);
+        break;
+      case "ArrowLeft":
+      case "ArrowUp":
+        e.preventDefault();
+        selectEnergyByIndex((here - 1 + energyOptions.length) % energyOptions.length);
+        break;
+      case "Home":
+        e.preventDefault();
+        selectEnergyByIndex(0);
+        break;
+      case "End":
+        e.preventDefault();
+        selectEnergyByIndex(energyOptions.length - 1);
+        break;
+    }
+  }
+
   let completedTasks = $derived.by(() => {
     let list = tasks.filter((t) => t.done);
     if (activeBucket !== "all") {
@@ -244,23 +294,25 @@
          from any bucket / list context. -->
     <div class="flex items-center gap-2 mr-2">
       <span class="text-[10px] text-text-tertiary hidden sm:inline" aria-hidden="true">I feel</span>
-      <div class="flex items-center gap-0.5 bg-bg-input border border-border-subtle rounded-lg p-0.5"
+      <div
+        bind:this={energyRadioGroupEl}
+        class="flex items-center gap-0.5 bg-bg-input border border-border-subtle rounded-lg p-0.5"
         role="radiogroup"
-        aria-label="My current energy">
-        {#each [
-          { value: null, label: '—' },
-          { value: 'high' as EnergyLevel, label: 'High' },
-          { value: 'medium' as EnergyLevel, label: 'Med' },
-          { value: 'brain_dead' as EnergyLevel, label: 'Low' },
-        ] as opt}
+        aria-label="My current energy"
+        tabindex="-1"
+        onkeydown={energyRadioKeydown}
+      >
+        {#each energyOptions as opt}
+          {@const checked = settings.currentEnergy === opt.value}
           <button
             class="text-[10px] px-2 py-0.5 rounded-md
-              {settings.currentEnergy === opt.value
+              {checked
                 ? 'bg-accent/15 text-accent'
                 : 'text-text-tertiary hover:text-text-secondary'}"
             role="radio"
-            aria-checked={settings.currentEnergy === opt.value}
+            aria-checked={checked}
             aria-label={opt.value ? `Set current energy to ${energyLabel(opt.value)}` : 'Clear current energy'}
+            tabindex={checked ? 0 : -1}
             onclick={() => cycleCurrentEnergy(opt.value)}
           >{opt.label}</button>
         {/each}