fix: rewrite let-chain conditionals for stable Rust

[FFFergie]

Summary

• Replace unstable let-chain conditionals with equivalent nested if let checks in four files.
• Keep behavior unchanged while restoring compatibility with stable Rust toolchains.
• Isolate this compatibility fix from Config validation gaps - overlapping ranges, empty ranges not caught #33 config validation changes.

Files

• src/app.rs
• src/process/ports.rs
• src/integrations/brew.rs
• src/integrations/docker.rs

Validation

• cargo check ✅
• cargo test ✅