ui: Add a "Default" option for the reasoning selector (#25846)

ui: Add a "Default" option for the reasoning selector (#25846)

* ui: add Default reasoning option that defers to the server

The webui always injected enable_thinking, overriding the chat template
default and the --reasoning flag, breaking models that reason
unconditionally (e.g. Gemma 4 E4B) on a fresh client.

Default sends nothing so the server decides, Off and effort levels
force the value as before. All choices are remembered.

Also remove the boolean thinking API from the conversations store and
drop ChatFormReasoningEffortSubmenu.svelte (dead code).

* ui: close the whole menu tree on reasoning level selection

The reasoning levels were raw buttons inside the SubContent, so
selecting one only closed the submenu via manual state while the root
dropdown stayed open. DropdownMenu.Item closes the full tree on select
like the sibling entries and brings native keyboard navigation.

* ui: prevent the add menu tooltip from flashing when the dropdown closes