Chore: fix clippy warnings for stable (1.87) and nightly (1.89)

[photovoltex]

Fixes the current clippy warnings and upcoming clippy warnings ( up until 1.79, aka current nightly). The fixes where applied automatically by clippy with the following commands.

cargo clippy --workspace --all-features --allow-dirty --fix

cargo +nightly clippy --workspace --all-features --allow-dirty --fix

[Copilot]

Pull Request Overview

This PR runs automatic cargo clippy --fix on stable (1.87) and nightly (1.89) to silence current and upcoming lints.

• Migrated explicit format!("…{}", var) and write!(…, "{}", var) calls to inline {var} syntax
• Replaced io::Error::new usages with the io::Error::other helper
• Simplified numeric expressions (e.g., removed redundant -1.0 *)

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/main.rs	Updated format strings to use inline {DISABLE_DISCOVERY} syntax
playback/src/mixer/mappings.rs	Simplified f64::powf call by removing redundant multiplication
oauth/src/lib.rs	Switched println! to inline {auth_url} interpolation
discovery/src/lib.rs	Updated error messages to use inline {name}/{error}
core/src/proxytunnel.rs	Replaced io::Error::new with io::Error::other and simplified map_err
core/src/mercury/types.rs	Changed write!(f, "{}", s) to write!(f, "{s}")