feat(frontend): add TipTap editor and Reasoning panel

Phase 2 implementation:
- Add TipTap editor with bold, italic, headings, lists, code blocks
- Add Reasoning panel with editable reasoning_type, confidence, steps
- Add Markdown to TipTap conversion on document load
- Add PUT /documents/{id}/content endpoint integration
- Add PATCH /documents/{id}/reasoning endpoint integration
- New types: ReasoningStep, ReasoningPanelData, TipTapContentResponse
- New store methods: updateReasoning, addReasoningStep, deleteReasoningStep
- New components: TipTapEditor.vue, ReasoningPanel.vue

package.json

@@ -9,6 +9,10 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "@tiptap/extension-code-block-lowlight": "^3.21.0",
+    "@tiptap/starter-kit": "^3.21.0",
+    "@tiptap/vue-3": "^3.21.0",
+    "lowlight": "^3.3.0",
     "pinia": "^2.1.7",
     "vue": "^3.4.21",
     "vue-router": "^4.3.0"