fix(io): Unicode-safe cv2 image IO + un-eat the [gpu] install hint (v0.6.6)

Two CLI/IO robustness bugs surfaced by issues #17 and #19.

#17 -- non-ASCII image paths (Chinese/Cyrillic/accented) failed on Windows:
cv2.imread/imwrite use the platform ANSI code-page API, so the decode came back
empty with a "can't open/read file" warning. New image_io.imread/imwrite route
through np.fromfile+cv2.imdecode / cv2.imencode+tofile (Unicode-safe, byte-
identical output, cv2.imread None-semantics preserved); all 8 cv2 read/write
call sites now go through it. Behavior-neutral on macOS/Linux (already accept
UTF-8 paths), so the fix is correct-by-construction for the Windows-only bug.

#19 (incidental) -- rich parsed the "[gpu]" in the GPU-extra install hint as a
style tag and dropped it, so the printed command was the un-installable
"pip install 'remove-ai-watermarks'". Escaped as \[gpu] at both call sites.

Tests: test_image_io.py (non-ASCII round-trip, alpha, missing/empty/garbage
semantics); test_cli.py::TestGpuHintMarkup (install hint keeps the extra).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_cli.py

@@ -509,3 +509,21 @@ class TestBatchCommand:
         assert result.exit_code == 0
         expected_dir = tmp_path / "input_clean"
         assert expected_dir.exists()
+
+
+class TestGpuHintMarkup:
+    """The GPU-extra install hint must survive rich markup (the ``[gpu]`` token
+    is otherwise parsed as a style tag and silently dropped)."""
+
+    def test_invisible_install_hint_keeps_gpu_extra(self, runner, sample_png):
+        with patch("remove_ai_watermarks.invisible_engine.is_available", return_value=False):
+            result = runner.invoke(main, ["invisible", str(sample_png)])
+        assert result.exit_code != 0
+        assert "remove-ai-watermarks[gpu]" in result.output
+
+    def test_all_install_hint_keeps_gpu_extra(self, runner, sample_png):
+        # The `all` pipeline skips the invisible step with a warning that carries
+        # the same hint; it must keep the [gpu] extra too.
+        with patch("remove_ai_watermarks.invisible_engine.is_available", return_value=False):
+            result = runner.invoke(main, ["all", str(sample_png)])
+        assert "remove-ai-watermarks[gpu]" in result.output