Make the size defaults and the chunk_size minimum 1000-based (closes #313)

The chunk_size and blob_size_limit defaults and the chunk_size minimum
were 1024-based, while the size parser reads 10MB as 10,000,000 bytes and
the docs and info use 1000-based units. So chunk_size: 1MB was rejected,
writing out a documented default changed it, and info showed an 11 GB
blob limit. They are now 1000-based like the parser. The examples below
the 1MB minimum in config.example.yml and the config init template are
replaced, and both now state the minimum.

Judgement call: s3.part_size keeps its 5MiB to 5GiB bounds, which are
S3's own limits, and MiB and GiB stay listed as accepted spellings.

Model: opus-5-5

internal/config/config.go

@@ -30,13 +30,15 @@ const appName = "vaultik"
 //nolint:gosec // G101: marker for detecting a pasted secret key, not a credential
 const secretKeyPrefix = "AGE-SECRET-KEY-"
 
-// Defaults and validation bounds for tunable settings.
+// Defaults and validation bounds for tunable settings. MB and GB are
+// 1000-based, as the size parser reads them; the S3 part sizes are S3's own
+// limits, which are 1024-based (MiB, GiB).
 const (
-	defaultBlobSizeLimit    = Size(10 * 1024 * 1024 * 1024) // 10GB
-	defaultChunkSize        = Size(10 * 1024 * 1024)        // 10MB
+	defaultBlobSizeLimit    = Size(10 * 1000 * 1000 * 1000) // 10GB
+	defaultChunkSize        = Size(10 * 1000 * 1000)        // 10MB
 	defaultS3PartSize       = Size(5 * 1024 * 1024)         // 5MiB
 	defaultCompressionLevel = 3
-	minChunkSize            = 1024 * 1024 // 1MB
+	minChunkSize            = 1000 * 1000 // 1MB
 	minCompressionLevel     = 1
 	maxCompressionLevel     = 19
 	// S3 accepts a multipart upload part from 5MiB to 5GiB.