⚡ Bolt: Inline duration string formatting to reduce heap allocations

[juntaochi]

💡 What:
Inlined duration formatting logic directly into format! macros in src/ui/mod.rs and removed the helper functions format_duration and format_duration_seconds which allocated intermediate strings.

🎯 Why:
The TUI draw loop runs frequently (~500ms intervals). Nested format! macro calls like format!("{} / {}", format_duration(...), format_duration(...)) generate multiple intermediate String objects allocated on the heap, which causes unnecessary overhead and GC/allocator pressure in a hot path.

📊 Impact:
Reduces String heap allocations by two per UI label during rendering loops.

🔬 Measurement:
A standalone benchmark script demonstrates the inlined formatting output exactly matches the previous logic, while completely avoiding intermediate string allocations. Passed all unit tests via cargo test.

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PR created automatically by Jules for task 10817319766327714170 started by @juntaochi

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