[6.x] Ability to save/reset changes to filter presets

[duncanmcclean]

This pull request brings back the ability to save/reset changes to listing filter presets, like you could in v5.

I've added a new selectedPreset state to track which preset the user selected (so it persists when the user adds/removes filters), separate from the activePreset state which tracks whether the current filters match any presets.

Filters are now cloned when selecting a preset to prevent mutation of saved preset data.

Closes #13754

[jasonvarga]

AI review:

Bug — Delete/Rename when dirty: When a preset is selected and the user changes filters, activePreset becomes null (filters no longer match). The dropdown (Rename/Delete) is still shown because it’s keyed by handle === selectedPreset. But deletePreset() uses activePreset.value (null), so it would remove the wrong preference key. renamePreset() uses activePresetPayload.value.display (i.e. presets.value[null]), which is undefined. saveExisting() uses activePreset.value in the key comparison, so the rename would not find the preset. These paths should use selectedPreset / selectedPresetPayload so Rename and Delete work when the view is dirty.

Suggested change:

1. Use selectedPreset for delete/rename flow
   When the dropdown is shown, the “current” preset is the one identified by selectedPreset, not activePreset. Update:

• deletePreset(): use selectedPreset.value (e.g. Statamic.$preferences.remove(\${preferencesKey.value}.${selectedPreset.value})`).
• renamePreset(): use selectedPresetPayload.value.display for the initial name (e.g. savingPresetName.value = selectedPresetPayload.value.display).
• saveExisting(): where you detect the preset to rename (key === activePreset.value), use key === selectedPreset.value (and keep using selectedPreset for any follow-up like selectPreset(savingPresetHandle.value)).
• presetPreferencesPayload: when used for renaming, the display fallback should come from the selected preset, e.g. activePresetPayload.value.display → selectedPresetPayload.value?.display ?? activePresetPayload.value?.display (or use selectedPresetPayload when in rename context) so the modal has the right name when dirty.
• Any other logic that means “the preset for this tab” in the dropdown context should use selectedPreset/selectedPresetPayload.

[duncanmcclean]

Just pushed some changes.

I've updated deletePreset(), renamePreset(), saveExisting(), and presetPreferencesPayload to use selectedPreset/selectedPresetPayload instead of activePreset/activePresetPayload.

I've also fixed the rename modal's "already exists" validation to filter by selectedPreset.