feat(cli): add --window-size flag for initial window dimensions

[HermeticOrmus]

Why

cosmic-term has no equivalent of xterm -geometry or alacritty --dimensions — users triggering terminals from keyboard shortcuts or scripts can't pre-size the window, and UI resizes don't persist across launches. This is the ask in #771.

What changed

src/main.rs gains a --window-size <WxH> flag (e.g. --window-size 1280x720), parsed from clap_lex alongside the existing --working-directory and --no-daemon handling.

When set, the value threads through libcosmic's Settings::size(iced::Size::new(width, height)) before cosmic::app::run, so it takes effect on first paint. When unset, behavior is unchanged (libcosmic default of 1024x768).

Both lower- and uppercase separator (x / X) accepted. Floating-point dimensions accepted. Invalid input (missing separator, non-numeric, zero, negative) exits with a one-line error.

How to test

cargo build --bin cosmic-term
./target/debug/cosmic-term --window-size 1280x720    # opens at the requested size
./target/debug/cosmic-term --window-size foo         # → "Invalid argument for --window-size: expected WxH, got \"foo\"", exit 1
./target/debug/cosmic-term --window-size             # → "Missing argument for --window-size", exit 1
./target/debug/cosmic-term --help                    # documents the new flag
cargo test --bin cosmic-term tests::                 # 6 new unit tests for parse_window_size

Notes

• Naming follows the issue ("window-size") and the existing flag style. --geometry was considered but xterm's geometry syntax (WxH+X+Y) implies position support, which would need a separate libcosmic API; staying with --window-size keeps the scope honest.
• Logical pixels (matching libcosmic's iced::Size), so the flag composes correctly with COSMIC_SCALE.
• 6 unit tests cover the parser: lower/uppercase separator, floats, missing separator, non-numeric, zero/negative.

Closes #771.

[jackpot51]

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