DRY: consolidate compact number formatting

[lowhung]

Summary

Extract compact numeric formatting (k/M suffix logic) into a shared formatting utility and reuse for rates/counts.

Why

format_rate and format_count each implement similar scale-threshold logic. Consolidating avoids divergence and improves formatting consistency.

Affected files

• src/ui/drawing.rs
• src/ui/panels.rs
• (new) src/ui/format.rs

Extraction plan

• Add shared utility in src/ui/format.rs (e.g. format_scaled).
• Refactor format_rate to delegate while preserving 0 => "" behavior.
• Refactor format_count to delegate while preserving integer-like output for small values.
• Add tests for thresholds, rounding, and zero behavior.

Risk

Medium. String output changes can affect tests/UX expectations and may require deliberate compatibility decisions.

Acceptance criteria

• Shared formatting utility exists and is reused by rate/count formatters.
• Existing expected output behavior is preserved or intentionally updated with tests.
• Tests pass.