Add --playback-speed CLI flag for controlling replay speed

[sh4wnbk]

Summary

Adds a --playback-speed CLI flag so users can set the initial replay speed from the command line.

Closes #36

Problem

playback_speed was hardcoded to 1 in main.py with no way to control it via CLI. Users could only change playback speed interactively after the replay launched.

Changes

• Added --playback-speed argument parsing in main.py, consistent with the existing --year and --round patterns
• Defaults to 1.0 if the flag is not provided

Usage

python main.py --viewer --year 2024 --round 12 --playback-speed 2.0
python main.py --viewer --year 2024 --round 5 --playback-speed 0.5

Note: Valid speed values are constrained by PLAYBACK_SPEEDS in race_replay.py ([0.1, 0.2, 0.5, 1.0, 2.0, 4.0, 8.0, 16.0, 32.0, 64.0, 128.0, 256.0]). Values not in this list will fall back to 1.0 — this behaviour is inherited from the existing window logic and unchanged by this PR.

[Copilot]

Pull request overview

Adds a new CLI flag to control the initial replay speed when launching the viewer, addressing the inability to set playback speed from the command line.

Changes:

• Added --playback-speed parsing in main.py
• Set default playback speed to 1.0 when the flag is omitted

[Copilot]

--playback-speed parsing can crash with IndexError (flag provided without a value) or ValueError (non-numeric value). Consider validating speed_index < len(sys.argv) and wrapping float(...) in try/except to print a helpful message and exit (or fall back to the default). Also consider keeping the default type consistent by using 1.0 throughout (the main() default is currently an int).