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 }