⚡ Bolt: Inline duration formatting to reduce allocations

[juntaochi]

💡 What: Inlines duration formatting logic directly into format! macros in App::update and removes the old string-returning helper functions (format_duration and format_duration_seconds).
🎯 Why: App::update is called frequently (every ~500ms). The previous approach allocated intermediate String objects via helper functions and passed them into an outer format! macro.
📊 Impact: Eliminates 4 intermediate String heap allocations per playback position update, reducing garbage generation in the TUI's hot path without changing the visual output.
🔬 Measurement: Verify via cargo build and cargo test --bin amcli ui to ensure the logic remains correct. The code compiles without warnings.

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PR created automatically by Jules for task 13573185752327949433 started by @juntaochi

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