feat(agent): add IRQ/SoftIRQ/Nice CPU breakdown and per-core frequency chart (Linux)

[svenvg93]

📃 Description

Adds richer CPU data to the system detail view: expanded breakdown categories (IRQ, SoftIRQ, Nice), a per-core frequency chart, and the advertised base clock speed in the CPU tooltip. The per-core frequency chart is currently Linux-only (reads from sysfs cpufreq). Base clock detection works on Linux.
closes #899

🪵 Changelog

➕ Added

• CPU breakdown chart now includes IRQ, SoftIRQ, and Nice time (indices 5–7 in the breakdown array; backwards-compatible with older hubs)
• Per-core CPU frequency chart in the CPU sheet (Linux only — requires cpufreq driver / sysfs)
• CPU base clock speed (MHz → GHz) shown in the CPU model tooltip in the system info bar
• cpu_mhz field added to system_details collection and migration

✏️ Changed

• CpuBreakdown slice extended from 5 to 8 elements
• system_details fields selector updated to include cpu_mhz

📷 Screenshots