chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/github.com/ProtonMail/go-crypto/openpgp/packet/padding.go

@@ -0,0 +1,26 @@
+package packet
+
+import (
+	"io"
+)
+
+// Padding type represents a Padding Packet (Tag 21).
+// The padding type is represented by the length of its padding.
+// see https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh#name-padding-packet-tag-21
+type Padding int
+
+// parse just ignores the padding content.
+func (pad Padding) parse(reader io.Reader) error {
+	_, err := io.CopyN(io.Discard, reader, int64(pad))
+	return err
+}
+
+// SerializePadding writes the padding to writer.
+func (pad Padding) SerializePadding(writer io.Writer, rand io.Reader) error {
+	err := serializeHeader(writer, packetPadding, int(pad))
+	if err != nil {
+		return err
+	}
+	_, err = io.CopyN(writer, rand, int64(pad))
+	return err
+}