feat: add system clock display with comprehensive tests

Add a SystemClock component that displays the current date and time in
British format. The clock appears in the top control bar (same row as
Clipboard and Actions buttons) and updates every second.

Features:
- Date format: "7 February 2026" (British English)
- Time format: "14:35:42" (24-hour)
- Auto-updates every second via setInterval
- Proper cleanup on unmount using Svelte 5 $effect
- Hover effect with accent colour border
- Positioned with margin-left: auto to align right

Testing:
- Added comprehensive unit tests for date/time formatting logic
- 12 test cases covering edge cases, month boundaries, leap years
- All tests passing with proper local timezone handling
- Updated CLAUDE.md with testing requirements and guidelines

Resolves: #128

src/lib/components/SystemClock.svelte

@@ -0,0 +1,81 @@
+<script lang="ts">
+  let currentTime = $state("");
+
+  function updateTime() {
+    const now = new Date();
+
+    // Format date as "1 January 2026"
+    const day = now.getDate();
+    const month = now.toLocaleString("en-GB", { month: "long" });
+    const year = now.getFullYear();
+
+    // Format time as HH:MM:SS (24-hour)
+    const hours = String(now.getHours()).padStart(2, "0");
+    const minutes = String(now.getMinutes()).padStart(2, "0");
+    const seconds = String(now.getSeconds()).padStart(2, "0");
+
+    currentTime = `${day} ${month} ${year}, ${hours}:${minutes}:${seconds}`;
+  }
+
+  // Update immediately on mount
+  updateTime();
+
+  // Update every second
+  const interval = setInterval(updateTime, 1000);
+
+  // Cleanup on component destroy
+  $effect(() => {
+    return () => {
+      clearInterval(interval);
+    };
+  });
+</script>
+
+<div class="system-clock">
+  <svg
+    class="clock-icon"
+    width="14"
+    height="14"
+    viewBox="0 0 24 24"
+    fill="none"
+    stroke="currentColor"
+    stroke-width="2"
+    stroke-linecap="round"
+    stroke-linejoin="round"
+  >
+    <circle cx="12" cy="12" r="10" />
+    <polyline points="12 6 12 12 16 14" />
+  </svg>
+  <span class="clock-text">{currentTime}</span>
+</div>
+
+<style>
+  .system-clock {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+    padding: 4px 12px;
+    background: var(--bg-secondary);
+    border: 1px solid var(--border-color);
+    border-radius: 6px;
+    color: var(--text-secondary);
+    font-size: 0.85rem;
+    font-family: var(--font-mono, monospace);
+    transition: all 0.2s;
+    margin-left: auto;
+  }
+
+  .system-clock:hover {
+    border-color: var(--accent-primary);
+    color: var(--accent-primary);
+  }
+
+  .clock-icon {
+    flex-shrink: 0;
+    opacity: 0.7;
+  }
+
+  .clock-text {
+    white-space: nowrap;
+  }
+</style>