Make new-thread flows respect project thread defaults #3

What to build

Update all new-thread entry points so they resolve the chosen Project's effective Project Thread Default before creating the thread. This includes the sidebar new-thread action and the Project Picker Flow (New thread in...).

When a Project has inherit global, the flow should continue to use the global default. When a Project has an explicit override, the flow should consistently use that Thread Workspace Mode.

Acceptance criteria

• The sidebar new-thread action uses the effective Project Thread Default for the chosen Project.
• The Project Picker Flow (New thread in...) uses the effective Project Thread Default for the chosen Project.
• inherit global continues to use the current global default behavior.
• Explicit project overrides take precedence over the global default in all supported new-thread flows.
• Automated tests cover both overridden and inherited behavior.

Blocked by

• Persist project thread defaults in server settings #1