Avoid startup stall with non-fatal startup preflight and cached betterdisplaycli resolution

[Copilot]

The daemon could appear stuck during launch before reaching early startup logs, with suspicion around get_betterdisplay_path() resolution. This change keeps startup resilient while making path discovery deterministic and efficient.

• Startup flow

  • Added a one-time betterdisplaycli preflight during App::initialize().
  • Preflight failures are logged (debug) and do not block startup.
  • Path resolution is cached and reused, so candidate paths are not re-evaluated on every call.

• Path resolution behavior

  • Refactored get_betterdisplay_path() to return anyhow::Result<PathBuf> instead of terminating the process.
  • Added strict override handling: if BETTERDISPLAYCLI_PATH is set but invalid, return a descriptive error.
  • Kept deterministic candidate checks (/opt/homebrew/bin, /usr/local/bin, /usr/bin, /bin) and removed PATH scanning from this flow.

pub fn set_input(input_code: u16, use_ddc_alt: bool) -> anyhow::Result<()> {
    let betterdisplay_path = get_betterdisplay_path()?;
    // ...
}

[Hacksore]

@copilot LGTM but can you remove the tests and i'll test it when im home?

[Copilot]

@copilot LGTM but can you remove the tests and i'll test it when im home?

Done in 2ccabbc — I removed the added path-resolution tests and kept the runtime fix changes intact.