refactor(duplicate-finder): modularize component

Break down the monolithic DuplicateFinder component into smaller, highly cohesive modules to improve maintainability and separation of concerns.

- Extract empty, loading, and intro states into dedicated components (`DuplicateFinderEmptyStates`).
- Extract the complex header and action logic into `DuplicateFinderHeader`.
- Move `DuplicateGroupCard` to its own dedicated component file.
- Relocate formatting utilities like byte conversion and relative time strings to a dedicated `format.ts` file.

src/components/duplicateFinder/DuplicateFinderEmptyStates.tsx

@@ -0,0 +1,36 @@
+export function DuplicateScanLoadingState({ progressLabel }: { progressLabel: string | null }) {
+  return (
+    <div className="flex flex-1 items-center justify-center gap-3 text-white/25">
+      <div className="h-5 w-5 animate-spin rounded-full border-2 border-white/15 border-t-white/50" />
+      <span className="text-sm">{progressLabel ? `${progressLabel}…` : "Preparing scan…"}</span>
+    </div>
+  );
+}
+
+export function DuplicateScanIntroState() {
+  return (
+    <div className="flex flex-1 items-center justify-center px-8">
+      <div className="max-w-sm text-center">
+        <svg className="mx-auto mb-4 h-10 w-10 text-white/10" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+          <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={1}
+            d="M8 16H6a2 2 0 01-2-2V6a2 2 0 012-2h8a2 2 0 012 2v2m-6 12h8a2 2 0 002-2v-8a2 2 0 00-2-2h-8a2 2 0 00-2 2v8a2 2 0 002 2z" />
+        </svg>
+        <p className="text-sm text-white/30">
+          Finds files with identical content regardless of filename or location.
+          Click <strong className="text-white/50">Scan for duplicates</strong> to begin.
+        </p>
+        <p className="mt-2 text-xs text-white/20">
+          Large libraries may take a minute — files are hashed from disk.
+        </p>
+      </div>
+    </div>
+  );
+}
+
+export function DuplicateScanEmptyState() {
+  return (
+    <div className="flex flex-1 items-center justify-center">
+      <p className="text-sm text-white/25">No duplicate files found.</p>
+    </div>
+  );
+}