Add transfer subcommand for bulk profile export/import

-e/--export writes every saved profile to a single TOML file as a
[[profile]] array; -i/--import reads one back and saves each entry,
failing on the first name collision rather than silently overwriting.
Reuses Profile's existing Serialize/Deserialize impl directly, so the
file format is just the same shape already written to
~/.config/porthole/profiles/<name>.toml, aggregated.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/cli.rs

@@ -41,6 +41,9 @@ pub enum Commands {
     #[command(visible_alias = "reset")]
     Wipe(WipeArgs),
 
+    /// Export saved profiles to a file, or import them from one.
+    Transfer(TransferArgs),
+
     /// Generate a shell completion script.
     Completions { shell: Shell },
 
@@ -182,3 +185,14 @@ pub struct WipeArgs {
     #[arg(short, long)]
     pub yes: bool,
 }
+
+#[derive(Args)]
+pub struct TransferArgs {
+    /// Export all saved profiles to a file.
+    #[arg(short, long, value_name = "PATH")]
+    pub export: Option<String>,
+
+    /// Import profiles from a file.
+    #[arg(short, long, value_name = "PATH")]
+    pub import: Option<String>,
+}