Default the CLI to the system time zone

It shipped with America/Denver hardcoded, taken from what dominates the
historical data on the server. That data is old; the reader has moved.
A zone belongs to whoever is looking at the calendar, and baking one in is
the same mistake as v1's offset-instead-of-zone in miniature -- it looks
right until the reader is somewhere else.

Falls back to UTC rather than to a populated guess: an obviously neutral
wrong answer gets noticed, a plausible one does not.

crates/runway-cli/Cargo.toml

@@ -11,6 +11,7 @@ runway-caldav = { workspace = true }
 chrono = { workspace = true }
 chrono-tz = { workspace = true }
 clap = { workspace = true }
+iana-time-zone = { workspace = true }
 tokio = { workspace = true }
 tracing = { workspace = true }
 tracing-subscriber = { workspace = true }

crates/runway-cli/src/main.rs

@@ -33,13 +33,28 @@ struct Cli {
     password: String,
 
     /// The zone to show times in, and to read floating times as.
-    #[arg(long, default_value = "America/Denver")]
+    ///
+    /// Defaults to whatever the machine is set to. Baking in a zone would be
+    /// the same mistake in miniature that this project exists to correct: the
+    /// right zone is a property of the reader, and readers move.
+    #[arg(long, default_value_t = local_timezone())]
     timezone: Tz,
 
     #[command(subcommand)]
     command: Command,
 }
 
+/// The system's own IANA zone, falling back to UTC when it cannot be read.
+///
+/// UTC rather than a guess at somewhere populated: a wrong zone that looks
+/// plausible is worse than an obviously neutral one, because nobody checks it.
+fn local_timezone() -> Tz {
+    iana_time_zone::get_timezone()
+        .ok()
+        .and_then(|name| name.parse().ok())
+        .unwrap_or(Tz::UTC)
+}
+
 #[derive(Subcommand)]
 enum Command {
     /// List the calendars the server offers.