feat(dc/tool-c): Add missing value imputation feature with 6 methods and MICE

Major features:
1. Missing value imputation (6 simple methods + MICE):
   - Mean/Median/Mode/Constant imputation
   - Forward fill (ffill) and Backward fill (bfill) for time series
   - MICE multivariate imputation (in progress, shape issue to fix)

2. Auto precision detection:
   - Automatically match decimal places of original data
   - Prevent false precision (e.g. 13.57 instead of 13.566716417910449)

3. Categorical variable detection:
   - Auto-detect and skip categorical columns in MICE
   - Show warnings for unsuitable columns
   - Suggest mode imputation for categorical data

4. UI improvements:
   - Rename button: "Delete Missing" to "Missing Value Handling"
   - Remove standalone "Dedup" and "MICE" buttons
   - 3-tab dialog: Delete / Fill / Advanced Fill
   - Display column statistics and recommended methods
   - Extended warning messages (8 seconds for skipped columns)

5. Bug fixes:
   - Fix sessionService.updateSessionData -> saveProcessedData
   - Fix OperationResult interface (add message and stats)
   - Fix Toolbar button labels and removal

Modified files:
Python: operations/fillna.py (new, 556 lines), main.py (3 new endpoints)
Backend: QuickActionService.ts, QuickActionController.ts, routes/index.ts
Frontend: MissingValueDialog.tsx (new, 437 lines), Toolbar.tsx, index.tsx
Tests: test_fillna_operations.py (774 lines), test scripts and docs
Docs: 5 documentation files updated

Known issues:
- MICE imputation has DataFrame shape mismatch issue (under debugging)
- Workaround: Use 6 simple imputation methods first

Status: Development complete, MICE debugging in progress
Lines added: ~2000 lines across 3 tiers

frontend-v2/src/modules/dc/pages/tool-c/components/Toolbar.tsx

@@ -113,7 +113,7 @@ const Toolbar: React.FC<ToolbarProps> = ({
       />
       <ToolbarButton
         icon={Trash2}
-        label="删除缺失值"
+        label="缺失值处理"
         colorClass="text-red-600 bg-red-50 hover:bg-red-100"
         onClick={onDropnaClick}
         disabled={!sessionId}
@@ -125,13 +125,6 @@ const Toolbar: React.FC<ToolbarProps> = ({
         onClick={onComputeClick}
         disabled={!sessionId}
       />
-      <ToolbarButton
-        icon={FileSearch}
-        label="去重"
-        colorClass="text-orange-600 bg-orange-50 hover:bg-orange-100"
-        onClick={onDedupClick}
-        disabled={true}
-      />
       
       <div className="w-[1px] h-8 bg-slate-200 mx-2"></div>
       
@@ -143,13 +136,6 @@ const Toolbar: React.FC<ToolbarProps> = ({
         onClick={onPivotClick}
         disabled={!sessionId}
       />
-      <ToolbarButton
-        icon={CalendarClock}
-        label="多重插补"
-        colorClass="text-rose-600 bg-rose-50 hover:bg-rose-100"
-        onClick={onMiceClick}
-        disabled={true}
-      />
       
       <div className="flex-1"></div>