chore: add totp feature gate (default-on) + data-encoding + qrcode deps

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

gui/Cargo.toml

@@ -8,6 +8,10 @@ license.workspace = true
 name = "authforge"
 path = "src/main.rs"
 
+[features]
+default = ["totp"]
+totp = ["dep:qrcode"]
+
 [dependencies]
 gtk = { package = "gtk4", version = "0.8" }
 adw = { package = "libadwaita", version = "0.6", features = ["v1_5"] }
@@ -16,3 +20,4 @@ anyhow = { workspace = true }
 zbus = { workspace = true }
 tokio = { workspace = true }
 futures-util = { workspace = true }
+qrcode = { workspace = true, optional = true }