Add "Latest Season + Future" option to the season picker

### Description

Currently the season picker only offers two extremes: pick individual seasons one by one, or use "All Seasons," which monitors every existing season (from S1) plus all future seasons in a single click. For long-running shows (anime with 100+ seasons, procedurals with 20+ seasons), that "All Seasons" click can queue an enormous back-catalog onto the download/storage pipeline.

The only current mitigation is disabling `allow_all_seasons` entirely, which also removes the future-seasons convenience and forces users into a season list that gets truncated to fit Discord's 25-option dropdown cap. That truncation currently keeps the *earliest* seasons, so for a very long-running show, users can't even see or request the season that's currently airing.

### Desired Behavior

Add a second option in the season picker, alongside "All Seasons" — something like "Latest Season" or "Latest + Future Seasons." Selecting it would:

- Monitor only the single most recent existing season (highest season_number, respecting allow_specials) instead of every season back to S1.
- Set monitor_new_items = All (the same mechanism "All Seasons" already uses) so future seasons keep getting picked up automatically.

This gives a middle ground for self-hosters who want to open requests to a wider group of users, without either (a) letting one click pull an entire back catalog, or (b) forcing manual per-season selection from a dropdown that may not even list the newest season.

Ideally gated behind its own config flag (e.g. `allow_latest_season`, mirroring the existing `allow_all_seasons`) so it can be enabled independently.

### Additional Context

Related to #18, but distinct — #18 asks to cap the picker to a single season generally; this is specifically about a third option next to "All Seasons" covering "latest + everything going forward," which is the common case for following an ongoing show without backfilling its full history.

Pointers into the code, for reference:
- `build_season_picker()` in `doplarr/src/providers/sonarr.rs` (~L234) — where the "All Seasons" sentinel option is added to the dropdown
- `ALL_SEASONS_ID` handling in `SelectedDetails::try_from` (~L462)
- Where `all_seasons` drives `monitor_new_items = Some(NewItemMonitorTypes::All)` for both existing (~L744) and new (~L818) series

A "latest season" sentinel could resolve at request time to `seasons.iter().map(|s| s.season_number).max()` (excluding specials unless `allow_specials`), then follow the same `monitor_new_items = All` path "All Seasons" already uses.

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