Export subcommands accept arbitrary positional args, then perform a Cronometer login #23

None of the export subcommands declare a Cobra Args: validator, so any number of extra positional arguments is silently accepted. Combined with #15 (format/flag validation happens after login), this means typos like crono-export bimoetrics --since today (or biometrics today instead of --since today) burn through the Cronometer login rate-limit budget.

Repro on HEAD 0c0e9b664850b334ac9851803f77b08553506af5 (binary at /tmp/qa-bin/crono-export):

$ /tmp/qa-bin/crono-export biometrics extra junk
error: login failed: failed to login: Too Many Attempts. Please try again later.
$ echo $?
1

Expected: extra positional args are rejected locally with a message like error: 'biometrics' takes no positional arguments and exit 1, without any network call.

Source: cmd/biometrics.go, cmd/exercises.go, cmd/nutrition.go, cmd/notes.go, cmd/servings.go — none set Args:.

Fix: add Args: cobra.NoArgs, to each export subcommand's &cobra.Command{...}.

Severity: minor (cosmetic on its own; major when combined with the rate-limit cost from #15).