feat(SliderArea): keyboard accessibility for the comparison slider

The slider element had role="slider" and tabindex="0" but no keyboard
handler — the focus ring appeared but the slider could not be moved.

Add a keydown handler implementing the standard ARIA slider contract:
- ArrowLeft / ArrowDown — step left by 1 percent
- ArrowRight / ArrowUp — step right by 1 percent
- Shift + arrow — coarse step (10 percent)
- PageUp / PageDown — coarse step (10 percent)
- Home — jump to 0
- End — jump to 100

Bounds and step sizes extracted as named constants (SLIDER_MIN,
SLIDER_MAX, SLIDER_STEP_FINE, SLIDER_STEP_COARSE). Position updates go
through sliderSpring.target so keyboard moves animate the same way as
pointer drags.

Also adds the missing ARIA attributes that screen readers need:
- aria-valuemin / aria-valuemax (bounds)
- aria-orientation (horizontal)

src/widgets/ComparisonView/ui/SliderArea/SliderArea.svelte

@@ -70,6 +70,19 @@ const SLIDER_PERSIST_DEBOUNCE_MS = 100;
 const SLIDER_PADDING_MOBILE_PX = 48;
 const SLIDER_PADDING_DESKTOP_PX = 96;
 
+/**
+ * Position bounds (percent of container width).
+ */
+const SLIDER_MIN = 0;
+const SLIDER_MAX = 100;
+
+/**
+ * Fine and coarse keyboard step sizes. Shift / Page keys use the coarse
+ * step; bare arrow keys use the fine step.
+ */
+const SLIDER_STEP_FINE = 1;
+const SLIDER_STEP_COARSE = 10;
+
 const fontA = $derived(comparisonStore.fontA);
 const fontB = $derived(comparisonStore.fontB);
 const isLoading = $derived(comparisonStore.isLoading || !comparisonStore.isReady);
@@ -130,6 +143,46 @@ function startDragging(e: PointerEvent) {
     handleMove(e);
 }
 
+/**
+ * Keyboard control for the comparison slider. Implements the standard
+ * ARIA slider keyboard contract: arrows step the position, Shift+arrow
+ * and PageUp/PageDown jump by the coarse step, Home/End snap to bounds.
+ */
+function handleKeydown(e: KeyboardEvent) {
+    const coarse = e.shiftKey;
+    const step = coarse ? SLIDER_STEP_COARSE : SLIDER_STEP_FINE;
+    const current = sliderSpring.target;
+    let next = current;
+
+    switch (e.key) {
+        case 'ArrowLeft':
+        case 'ArrowDown':
+            next = current - step;
+            break;
+        case 'ArrowRight':
+        case 'ArrowUp':
+            next = current + step;
+            break;
+        case 'PageDown':
+            next = current - SLIDER_STEP_COARSE;
+            break;
+        case 'PageUp':
+            next = current + SLIDER_STEP_COARSE;
+            break;
+        case 'Home':
+            next = SLIDER_MIN;
+            break;
+        case 'End':
+            next = SLIDER_MAX;
+            break;
+        default:
+            return;
+    }
+
+    e.preventDefault();
+    sliderSpring.target = Math.max(SLIDER_MIN, Math.min(SLIDER_MAX, next));
+}
+
 const storeSliderPosition = debounce((value: number) => {
     comparisonStore.sliderPosition = value;
 }, SLIDER_PERSIST_DEBOUNCE_MS);
@@ -271,8 +324,12 @@ const paddingClass = $derived(
                     role="slider"
                     tabindex="0"
                     aria-valuenow={Math.round(sliderPos)}
+                    aria-valuemin={SLIDER_MIN}
+                    aria-valuemax={SLIDER_MAX}
+                    aria-orientation="horizontal"
                     aria-label="Font comparison slider"
                     onpointerdown={startDragging}
+                    onkeydown={handleKeydown}
                     class="
                         relative w-full max-w-6xl h-full
                         flex flex-col justify-center