Add advanced scheduler features (time limits, group chaining, group start/end triggers)

[miladsh02]

Context

Most download manager applications include a scheduler with more advanced
capabilities than what we currently support — for example:

• Setting a time limit for a download group (pause/stop after X time)
• Triggering an action when a group finishes downloading
• Scheduling when a group should start downloading

While testing our current scheduler, it became clear these features would
significantly improve usability, especially for users managing multiple
download groups.

Proposed Features

• Time-limited groups — allow a group to be capped at a set duration,
  after which it auto-pauses or stops
• Post-completion actions — let users define what happens when a group
  finishes (e.g. start another group, shut down, send notification)
• Scheduled group start — allow a group to be set to begin downloading
  at a specific date/time

Why this matters

These are common patterns in established download managers, and without them
our scheduler feels limited for anyone managing more than a single group at a time.

Open questions

• Should these settings live per-group or as global scheduler defaults?
• Do we need a UI for chaining multiple groups (A finishes → B starts)?

[miladsh02]

@PureJoyMind
It's good to have enhancement label on this issue