TASK-026 TUI: Network I/O history sparkline on System page

Description

The CPU sparkline tracks 60 samples of CPU history. Add equivalent sparklines for network bytes-in and bytes-out on the System page.

Implementation:

• Add net_down_history: deque[float] and net_up_history: deque[float] (maxlen=60) to SystemStats
• Append down and up values in sample() (use 0.0 when delta is unavailable)
• Add two Sparkline widgets in a row below the CPU sparkline or beside it: "Net ↓" and "Net ↑"
• Border titles show current rate: "Net ↓ 1.2 MB/s"
• Match the existing sparkline styling (color: $accent, round border)
• Update via the same _apply_system() path

Acceptance Criteria

• #1 Two sparklines (download, upload) visible on System page
• #2 Border titles show current rate in human-readable format
• #3 History tracks 60 samples at SYS_REFRESH_SECS interval
• #4 Layout doesn't push process table off screen