Save custom snooze durations to the snooze menu

[fagnersales]

Problem

Custom… (from #11210) lets you snooze for any duration, but it doesn't remember the choice. If you regularly snooze for something the menu doesn't offer, like 45 minutes or 2 days, you have to open the dialog and type it again every time.

Proposal

Add an Add to snooze list checkbox to the Custom snooze dialog's Duration mode. When it's ticked, the duration is saved and appears in every snooze menu (the hover clock, the thread context menu, bulk snooze and the chat header), below the built-in options and above Custom….

• Only durations can be saved. Date and time mode has no checkbox, so a saved entry never goes stale.
• The list keeps the 5 most recent. Saving the same duration again moves it to the top instead of adding a copy.
• Hovering a saved entry shows an X that removes it.

It's client-side only. thread.snooze already accepts any ISO snoozedUntil, so nothing changes on the server or in the protocol.

Relationship to #11399

This doesn't overlap with configurable presets. #11399 changes the built-in times once, in settings ("my morning is 07:00"). This keeps the one-off durations you actually use so you can pick them again. Both could ship.

I have a working web/desktop implementation (~190 lines plus tests) and I'm happy to open a PR if this is wanted.

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Prepared by Claude Opus 5.5 on behalf of @fagnersales.

[plasalviar]

+1. A lighter variant: remember the last custom duration automatically and show it as a quick pick above Custom…, without a checkbox. Most of the time I just repeat the custom snooze I used last.