feat: image upload optimization with sharp, thumbnails, and structured error codes (#304)

- Add sharp for server-side image processing (WebP conversion + thumbnails)
- New shared backend utility for image upload, optimization, and cleanup
- Return structured error codes from upload endpoints (IMAGE_TOO_LARGE, INVALID_TYPE, etc.)
- Frontend error code mapping with i18n support (EN + DE)
- MedicationAvatar tries thumbnail first, falls back to full image
- Error display in MedicationsPage, MobileEditModal, and Auth avatar upload

Closes #302

frontend/src/test/components/Auth.test.tsx

@@ -896,7 +896,7 @@ describe("AuthProvider methods", () => {
 		});
 
 		const file = new File(["avatar"], "avatar.png", { type: "image/png" });
-		await expect(result.current.uploadAvatar(file)).rejects.toThrow("Upload failed");
+		await expect(result.current.uploadAvatar(file)).rejects.toThrow("UNKNOWN");
 	});
 
 	it("deleteAvatar succeeds and refreshes user", async () => {