fix: bail on --watch without a terminal

[Mathjk]

Fixes #982.

Bug

procs -w with non-terminal stdout enters the TUI watch loop and emits ANSI frames into the pipe/file forever. A related case: procs -w </dev/null makes the getch keyboard thread spin at ~100% CPU (stdin.read returns Ok(0) instantly in a tight loop) and q can't quit.

Plain procs | cat is correct and unchanged — only watch mode is broken on non-tty.

Fix (+14/-1, src/main.rs)

In run_watch — before any frame write or Watcher::start:

if !stdout().is_terminal() {
    return Err(Error::msg("--watch requires a terminal, but stdout is not a TTY"));
}
if !std::io::stdin().is_terminal() {
    return Err(Error::msg("--watch requires a terminal, but stdin is not a TTY"));
}

IsTerminal is std (≥1.70; crate MSRV 1.88) — no dependency churn. Covers -w, --watch, and -W/--watch-interval (all funnel here). Bails through the existing error: stderr printer → exit 1.

Verified

Case	Before	After
procs -w | cat / > file	ANSI frames forever	error: --watch requires a terminal…, rc=1
procs -w </dev/null (tty out)	getch ~100% CPU spin	error: … stdin is not a TTY, rc=1
procs -w | head	late "Broken pipe"	immediate clean bail
procs | cat	table output	unchanged
procs -w real tty	renders, q quits	unchanged (verified Linux PTY + Windows ConPTY)

cargo test 24/24; fmt/clippy clean; x86_64-pc-windows-gnu build green.