TASK-027 TUI: Sort process table by memory with m key

Description

The process table on the System page is always sorted by CPU%. Add a toggle so pressing m when on the System page (or when the process table is focused) switches sort between CPU% (default) and RSS memory.

Implementation:

• Add _proc_sort: str = "cpu" to DashboardApp (or a SystemPage widget if pages are refactored)
• Bind m scoped to the system page: action_toggle_proc_sort()
• _apply_system() sorts procs by the current sort field before adding rows
• Process table border title reflects active sort: "Processes [CPU]" or "Processes [Mem]"
• m key hint visible in the table border or Footer

Acceptance Criteria

• #1 m toggles process table sort between CPU% and RSS memory
• #2 Current sort mode visible in table border title
• #3 Sort persists until toggled again or page changes
• #4 Table re-sorts immediately on keypress without waiting for next sample