front: convert number to Duration where appropriate

[emersion]

Description and goal

Many types and functions still use number to store durations. Now that we have a proper Duration type (see #8816), it would be nice to convert these number values to Duration.

Note, we can't do that for auto-generated types.

• StdcmPathStep.tolerances (front: use Duration in StdcmPathStep tolerances #10914)
• StdcmPathStep.stopFor
• ValidStdcmConfig.{gridMarginBefore,gridMarginAfter} (front: use Duration for OsrdStdcmConfState grid margins #10916)
• StdcmResultsOperationalPoint.duration
• TimesStopsRow
• secondsToTimeString() and timeStringToSeconds()
• TODO: find more

Acceptance criteria

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