Name the setting and value when a size setting does not parse (closes #429)

A non-numeric chunk_size or blob_size_limit failed with an error that named neither the setting nor the value. Every size setting now fails at load with its name and the quoted value, for example chunk_size: "banana" is not a size; use a number of bytes or a unit such as 10MB. A value too large for a size says so, with the setting name.

Model: opus-5-5

internal/cli/config_test.go

@@ -252,7 +252,7 @@ func TestConfigSetKeepsFileThatWouldNotLoad(t *testing.T) {
 		{configWithoutRecipients, "snapshots.home.paths", "/srv",
 			"into []string"},
 		{configWithoutRecipients, "chunk_size", "fast",
-			"invalid size format"},
+			`chunk_size: "fast" is not a size`},
 		// Values of the right type that fail validation.
 		{configWithoutRecipients, "compression_level", "99",
 			"compression_level must be between 1 and 19"},