perf: remove redundant render clones

[matheuseabra]

What changed?

• Remove the redundant SparklineBar clone on every render by moving the owned vector into Ratatui.
• Avoid cloning the minimum-size title line when rendering narrow panes.
• Remove the unused Config: Clone implementation.
• Sharpen unit-test names and coverage for compact formatting, percentages, gradients, and all metric labels/icons.

This applies the installed Rust best-practice guidance without adding abstractions, dependencies, or changing the UI contract.

Validation

• cargo fmt --all -- --check
• cargo check --locked
• cargo test --locked (7 passed)
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo clippy --locked --all-targets --all-features -- -D warnings -D clippy::perf
• No UI surface changed; rendered terminal verification was not needed.

Notes for reviewers

The review also checked error handling, unsafe code, ownership, trait-object use, and documentation applicability. The existing Result/Option handling and small binary architecture were retained because no concrete improvement was supported by the code or lint output.