fix(build): move EstimateBackupSize to platform-independent file

Fixes Windows, OpenBSD, and NetBSD builds by extracting
EstimateBackupSize from disk_check.go (which has build tags
excluding those platforms) to a new estimate.go file.

internal/checks/disk_check.go

@@ -109,28 +109,3 @@ func FormatDiskSpaceMessage(check *DiskSpaceCheck) string {
 
 	return msg
 }
-
-// EstimateBackupSize estimates backup size based on database size
-func EstimateBackupSize(databaseSize uint64, compressionLevel int) uint64 {
-	// Typical compression ratios:
-	// Level 0 (no compression): 1.0x
-	// Level 1-3 (fast): 0.4-0.6x
-	// Level 4-6 (balanced): 0.3-0.4x
-	// Level 7-9 (best): 0.2-0.3x
-
-	var compressionRatio float64
-	if compressionLevel == 0 {
-		compressionRatio = 1.0
-	} else if compressionLevel <= 3 {
-		compressionRatio = 0.5
-	} else if compressionLevel <= 6 {
-		compressionRatio = 0.35
-	} else {
-		compressionRatio = 0.25
-	}
-
-	estimated := uint64(float64(databaseSize) * compressionRatio)
-
-	// Add 10% buffer for metadata, indexes, etc.
-	return uint64(float64(estimated) * 1.1)
-}