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/format.ts

@@ -0,0 +1,29 @@
+import { DuplicateScanProgress } from "../../store";
+
+export function formatBytes(bytes: number): string {
+  if (bytes >= 1_073_741_824) return `${(bytes / 1_073_741_824).toFixed(1)} GB`;
+  if (bytes >= 1_048_576) return `${(bytes / 1_048_576).toFixed(1)} MB`;
+  if (bytes >= 1024) return `${(bytes / 1024).toFixed(0)} KB`;
+  return `${bytes} B`;
+}
+
+export function formatRelativeTime(unixSecs: number): string {
+  const diff = Math.floor(Date.now() / 1000) - unixSecs;
+  if (diff < 60) return "just now";
+  if (diff < 3600) return `${Math.floor(diff / 60)}m ago`;
+  if (diff < 86400) return `${Math.floor(diff / 3600)}h ago`;
+  return `${Math.floor(diff / 86400)}d ago`;
+}
+
+export function duplicateProgressLabel(progress: DuplicateScanProgress | null): string | null {
+  if (!progress) return null;
+  if (progress.phase === "checking") return "Checking file sizes";
+  if (progress.phase === "hashing") return "Hashing duplicate candidates";
+  return "Confirming exact matches";
+}
+
+export function duplicateProgressPercent(progress: DuplicateScanProgress | null): number {
+  return progress && progress.total > 0
+    ? Math.round((progress.processed / progress.total) * 100)
+    : 0;
+}