feat: add --review flag with browser-based side-by-side image review

- Launches local HTTP server with dark-themed review UI
- Side-by-side image comparison per duplicate group
- Checkbox selection + delete confirmation
- Shows file size and path per image
- Exact/similar badges per group
- Shutdown endpoint for clean exit
- Magic byte format detection (fixes misnamed screenshots)
- 23 tests passing

tests/image_phase.rs

@@ -146,3 +146,16 @@ fn misnamed_jpeg_as_png_still_scanned() {
     let fake = fake.unwrap();
     assert_eq!(orig.sha256, fake.sha256, "same content = same sha256");
 }
+
+#[test]
+fn cli_review_flag_accepted_no_dupes() {
+    let bin = env!("CARGO_BIN_EXE_deduper");
+    let output = std::process::Command::new(bin)
+        .arg("/a0/usr/projects/deduper/.a0proj/test_media/images/empty_dir")
+        .arg("--review")
+        .output()
+        .expect("failed to run deduper binary");
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    assert!(output.status.success(), "--review on empty dir should exit 0: {}", String::from_utf8_lossy(&output.stderr));
+    assert!(stdout.contains("no image duplicates found"), "should report no dupes: {stdout}");
+}