feat: add contrast/sharpen strength controls, disable binarization

Major improvements to preprocessing controls:

Backend:
- Add contrast_strength (0.5-3.0) and sharpen_strength (0.5-2.0) to PreprocessingConfig
- Auto-detection now calculates optimal strength based on image quality metrics:
  - Lower contrast → Higher contrast_strength
  - Lower edge strength → Higher sharpen_strength
- Disable binarization in auto mode (rarely beneficial)
- CLAHE clipLimit now scales with contrast_strength
- Sharpening uses unsharp mask with variable strength

Frontend:
- Add strength sliders for contrast and sharpen in manual mode
- Sliders show current value and strength level (輕微/正常/強/最強)
- Move binarize option to collapsible "進階選項" section
- Updated i18n translations for strength labels

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

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

backend/app/schemas/task.py

@@ -72,13 +72,25 @@ class PreprocessingConfig(BaseModel):
         default=PreprocessingContrastEnum.CLAHE,
         description="Contrast enhancement method"
     )
+    contrast_strength: float = Field(
+        default=1.0,
+        ge=0.5,
+        le=3.0,
+        description="Contrast enhancement strength (0.5=subtle, 1.0=normal, 2.0=strong, 3.0=maximum)"
+    )
     sharpen: bool = Field(
         default=True,
         description="Enable sharpening for faint lines"
     )
+    sharpen_strength: float = Field(
+        default=1.0,
+        ge=0.5,
+        le=2.0,
+        description="Sharpening strength (0.5=subtle, 1.0=normal, 1.5=strong, 2.0=maximum)"
+    )
     binarize: bool = Field(
         default=False,
-        description="Enable binarization (aggressive, for very low contrast)"
+        description="Enable binarization (aggressive, for very low contrast). Not recommended for most documents."
     )