[feature]: bandwidth monitoring per interface

[karol-broda]

Problem / Use Case

no way to see real time bandwidth usage per network interface from snitch

Proposed Solution

• snitch bw cli subcommand for standalone bandwidth monitoring (table, json, csv output)
• b keybinding in tui to toggle bandwidth overlay
• platform support for linux (/proc/net/dev) and macos (sysctl)

Area

CLI, TUI (interactive mode)

Additional Context

wip on feat/bandwidth-monitor branch. cli subcommand and data layer are done, tui rendering still pending