feat: improve remove background with edge smoothing, color decontamination, output formats

- Expose birefnet-hr-matting in UI (People/Ultra) and fix model defaults
  (People/Max now uses birefnet-matting for true alpha matting)
- Add output format selector (PNG/WebP/AVIF) with lossless alpha support
- Add edge smoothing post-processing (Off/Light/Medium/Strong) via
  morphological mask refinement to reduce gray halo artifacts
- Add color decontamination to remove background color spill from
  semi-transparent edge pixels
- Thread new settings through full stack: frontend -> API schema ->
  Python sidecar -> Sharp effects pipeline
- Add i18n keys for all 21 locales
- Add unit tests for new option serialization (3 tests)
- Add integration tests for new settings validation (4 tests)

tests/unit/ai/background-removal.test.ts

@@ -97,6 +97,35 @@ describe("removeBackground", () => {
       });
     });
 
+    it("serializes edgeRefine option into the args JSON", async () => {
+      await removeBackground(FAKE_INPUT, FAKE_OUTPUT_DIR, { edgeRefine: 2 });
+
+      const args = vi.mocked(runPythonWithProgress).mock.calls[0][1];
+      expect(JSON.parse(args[2])).toEqual({ edgeRefine: 2 });
+    });
+
+    it("serializes decontaminate option into the args JSON", async () => {
+      await removeBackground(FAKE_INPUT, FAKE_OUTPUT_DIR, { decontaminate: true });
+
+      const args = vi.mocked(runPythonWithProgress).mock.calls[0][1];
+      expect(JSON.parse(args[2])).toEqual({ decontaminate: true });
+    });
+
+    it("serializes all post-processing options together", async () => {
+      await removeBackground(FAKE_INPUT, FAKE_OUTPUT_DIR, {
+        model: "birefnet-matting",
+        edgeRefine: 1,
+        decontaminate: true,
+      });
+
+      const args = vi.mocked(runPythonWithProgress).mock.calls[0][1];
+      expect(JSON.parse(args[2])).toEqual({
+        model: "birefnet-matting",
+        edgeRefine: 1,
+        decontaminate: true,
+      });
+    });
+
     it("converts input to PNG via sharp before writing to disk", async () => {
       await removeBackground(FAKE_INPUT, FAKE_OUTPUT_DIR);