refactor!: disk-first zero-copy architecture with file-to-file cwebp conversion

BREAKING CHANGE: Complete refactoring of the conversion pipeline.

- Replace in-memory Page/PageContainer with disk-based PageFile struct
- Extract archives to temp directory instead of loading into memory
- Use cwebp InputFile/OutputFile for zero-copy file-to-file conversion
- Use cwebp -crop for splitting (no Go-side image decode needed)
- Check if already converted before extracting (fast pre-check)
- Remove oliamb/cutter dependency (replaced by cwebp -crop)
- Remove page_container.go (no longer needed)
- Add comprehensive tests with improved coverage

Co-authored-by: Belphemur <197810+Belphemur@users.noreply.github.com>

pkg/converter/webp/webp_provider.go

@@ -2,8 +2,6 @@ package webp
 
 import (
 	"fmt"
-	"image"
-	"io"
 	"strings"
 	"sync"
 
@@ -37,10 +35,23 @@ func PrepareEncoder() error {
 	return nil
 }
 
-func Encode(w io.Writer, m image.Image, quality uint) error {
+// EncodeFile converts an image file directly to WebP using cwebp.
+// This is a zero-copy operation: no image data is loaded into Go memory.
+func EncodeFile(inputPath string, outputPath string, quality uint) error {
 	return webpbin.NewCWebP(config).
 		Quality(quality).
-		InputImage(m).
-		Output(w).
+		InputFile(inputPath).
+		OutputFile(outputPath).
+		Run()
+}
+
+// EncodeFileWithCrop converts a cropped region of an image file to WebP.
+// Uses cwebp's native -crop flag — no Go-side image decode needed.
+func EncodeFileWithCrop(inputPath string, outputPath string, quality uint, x, y, width, height int) error {
+	return webpbin.NewCWebP(config).
+		Quality(quality).
+		Crop(x, y, width, height).
+		InputFile(inputPath).
+		OutputFile(outputPath).
 		Run()
 }