Print progress at once off a terminal, keep warnings out of redraws (closes #13)

When stderr is not a terminal, each phase prints its zero-state line
as it starts instead of after its first item. stderrIsTTY uses
term.IsTerminal from golang.org/x/term, now a direct dependency, so
/dev/null is no longer taken for a terminal.

A spinner keeps the library's background redraw, so its count and
elapsed time stay current while a phase waits for its next item. A
warning printed during a spinner phase goes through the bar
(progressbar.Bprintln), which prints it before its next redraw instead
of racing it. Bars with a total have no background redraw and still
print warnings directly.

Model: opus-5-5

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- progress prints at once on a non-terminal, uses a real terminal test,
+  and prints warnings through a spinner instead of racing its redraw
+  (2026-10-03, https://git.eeqj.de/sneak/sfdupes/issues/13)
+
 - `scan` holds a lock on a lock file beside the database for its whole run,
   so a second `scan` fails at once with exit 1 (2026-10-03,
   https://git.eeqj.de/sneak/sfdupes/issues/53)