fix: satisfy current Clippy lint for brightness icon

[FoOcean]

Pre-submission checklist

• I have read CONTRIBUTING.md and completed the required checks
• Local and CI checks pass (local checks pass; CI is pending)
• I have completed a self-review
• I confirm that I haven't read the checkboxes and just ticked them all

Change type (select one)

• feat New feature
• fix Bug fix
• docs Documentation or templates
• style Formatting with no behavior change
• refactor Code change that neither fixes a bug nor adds a feature
• perf Performance improvement
• test Test-related change
• chore Build, CI, dependency, or tooling change
• revert Revert
• security Security fix

Scope (select all that apply)

• Application core
• UI style or layout
• Application behavior
• Dependency changes
• Plugin API
• CI/CD configuration
• Documentation
• Other (describe below)

Related issues

Follow-up to #188.

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Description

Summary

Use the current Clippy-recommended four-byte chunk iteration for brightness icon pixels.

Motivation and context

The GitHub Actions runner uses Rust 1.98, which rejects chunks_exact_mut(4) with -D warnings. The locally installed Rust 1.96 did not flag it, so this one-line compatibility fix is needed before the 1.3.9 release workflow runs.

Changes

• Replace chunks_exact_mut(4) with as_chunks_mut::<4>().0 in the brightness icon conversion loop.

UI changes

No visible UI changes.

Validation: cargo fmt --all, cargo clippy --workspace --offline -- -D warnings, and cargo build --release --offline passed locally. The prior GitHub Actions log showed only the Rust 1.98 lint error addressed here.