feat: Migrate to React Flow and add Fixed + Dynamic category mode

Frontend:
- Migrate MindmapDAG from D3.js to React Flow (@xyflow/react)
- Add custom node components (QueryNode, CategoryHeaderNode, AttributeNode)
- Add useDAGLayout hook for column-based layout
- Add "AI" badge for LLM-suggested categories
- Update CategorySelector with Fixed + Dynamic mode option
- Improve dark/light theme support

Backend:
- Add FIXED_PLUS_DYNAMIC category mode
- Filter duplicate category names in LLM suggestions
- Update prompts to exclude fixed categories when suggesting new ones
- Improve LLM service with better error handling and logging
- Auto-remove /no_think prefix for non-Qwen models
- Add smart JSON format detection for model compatibility
- Improve JSON extraction with multiple parsing strategies

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/package-lock.json

@@ -10,6 +10,7 @@
       "dependencies": {
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+        "@xyflow/react": "^12.9.3",
         "antd": "^6.0.0",
         "d3": "^7.9.0",
         "react": "^19.2.0",
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-      "dev": true,
+      "devOptional": true,
       "license": "MIT",
       "peer": true,
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@@ -2763,6 +2764,38 @@
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+    "node_modules/@xyflow/react": {
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+      "resolved": "https://registry.npmjs.org/@xyflow/react/-/react-12.9.3.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "@xyflow/system": "0.0.73",
+        "classcat": "^5.0.3",
+        "zustand": "^4.4.0"
+      },
+      "peerDependencies": {
+        "react": ">=17",
+        "react-dom": ">=17"
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+    "node_modules/@xyflow/system": {
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+      "resolved": "https://registry.npmjs.org/@xyflow/system/-/system-0.0.73.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "@types/d3-drag": "^3.0.7",
+        "@types/d3-interpolate": "^3.0.4",
+        "@types/d3-selection": "^3.0.10",
+        "@types/d3-transition": "^3.0.8",
+        "@types/d3-zoom": "^3.0.8",
+        "d3-drag": "^3.0.0",
+        "d3-interpolate": "^3.0.1",
+        "d3-selection": "^3.0.0",
+        "d3-zoom": "^3.0.0"
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@@ -3001,6 +3034,12 @@
         "url": "https://github.com/chalk/chalk?sponsor=1"
       }
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+    "node_modules/classcat": {
+      "version": "5.0.5",
+      "resolved": "https://registry.npmjs.org/classcat/-/classcat-5.0.5.tgz",
+      "integrity": "sha512-JhZUT7JFcQy/EzW605k/ktHtncoo9vnyW/2GspNYwFlN1C/WmjuV/xtS04e9SOkL2sTdw0VAZ2UGCcQ9lR6p6w==",
+      "license": "MIT"
+    },
     "node_modules/classnames": {
       "version": "2.5.1",
       "resolved": "https://registry.npmjs.org/classnames/-/classnames-2.5.1.tgz",
@@ -4809,6 +4848,15 @@
         "punycode": "^2.1.0"
       }
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+    "node_modules/use-sync-external-store": {
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+      "resolved": "https://registry.npmjs.org/use-sync-external-store/-/use-sync-external-store-1.6.0.tgz",
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+      "license": "MIT",
+      "peerDependencies": {
+        "react": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0"
+      }
+    },
     "node_modules/vite": {
       "version": "7.2.6",
       "resolved": "https://registry.npmjs.org/vite/-/vite-7.2.6.tgz",
@@ -4954,6 +5002,34 @@
       "peerDependencies": {
         "zod": "^3.25.0 || ^4.0.0"
       }
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+    "node_modules/zustand": {
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+      "resolved": "https://registry.npmjs.org/zustand/-/zustand-4.5.7.tgz",
+      "integrity": "sha512-CHOUy7mu3lbD6o6LJLfllpjkzhHXSBlX8B9+qPddUsIfeF5S/UZ5q0kmCsnRqT1UHFQZchNFDDzMbQsuesHWlw==",
+      "license": "MIT",
+      "dependencies": {
+        "use-sync-external-store": "^1.2.2"
+      },
+      "engines": {
+        "node": ">=12.7.0"
+      },
+      "peerDependencies": {
+        "@types/react": ">=16.8",
+        "immer": ">=9.0.6",
+        "react": ">=16.8"
+      },
+      "peerDependenciesMeta": {
+        "@types/react": {
+          "optional": true
+        },
+        "immer": {
+          "optional": true
+        },
+        "react": {
+          "optional": true
+        }
+      }
     }
   }
 }