feat: make key repeat delay and interval configurable

[rengers]

Summary

On macOS and Windows the OS does not auto-repeat synthetic key events, so
lan-mouse regenerates key repeats itself. The repeat delay and interval were
hardcoded to 500 ms / 32 ms, which can feel off compared to the sender's own
keyboard settings. This makes both values configurable, defaulting to the
current behaviour when unset.

Changes

• input-emulation: add EmulationOptions { key_repeat_delay, key_repeat_interval } (with Default = 500 ms / 32 ms, exposed via
  DEFAULT_KEY_REPEAT_DELAY / DEFAULT_KEY_REPEAT_INTERVAL) and thread it
  through InputEmulation::new / with_backend into the macOS and Windows
  backends, replacing the per-backend hardcoded constants. The other backends
  leave key repeat to the OS and ignore the value (guarded so there are no
  unused-variable warnings on those targets).
• config: add optional key_repeat_delay / key_repeat_interval
  (milliseconds) and a Config::emulation_options() accessor that maps them
  onto EmulationOptions, falling back to the defaults per field.
• docs: document the options in config.toml and the README.

There is no behaviour change unless the new keys are set.

# milliseconds; only the macOS and Windows backends use these
key_repeat_delay = 300
key_repeat_interval = 20

Test plan

• cargo fmt --all — clean
• cargo clippy -p input-emulation --no-default-features --all-targets -- -D warnings — clean
• root crate cargo check --no-default-features — clean (macOS)
• CI: cargo fmt / clippy / test --workspace --all-targets --all-features
  on Linux + the Windows emulation backend. I could not build --all-features
  locally: the gtk frontend pulls in libadwaita, which does not link on my
  macOS/nix toolchain, so the Linux backends and the Windows path are only
  compile-checked by mirroring the macOS change.