[Bug]: "Add project starts in" saved on the All environments scope copies one machine's path to every machine #35115

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/web

Steps to reproduce

1. Connect a macOS desktop app to a remote Linux environment, so Settings has two environments.
2. Open Settings → New projects and leave the scope selector on its default, All environments.
3. Set Add project starts in to a Mac folder, e.g. /Users/me/code/.
4. Open Add Project and choose the Linux environment.

Expected behavior

The Linux environment's picker opens in a folder that exists on Linux. That could be its own value or ~/. A path that only exists on another machine shouldn't reach the Linux server.

Actual behavior

The Linux picker opens at /Users/me/code/, which doesn't exist there. You have to clear the field back to / and browse down to your projects every time.

The Linux server's own ~/.t3/userdata/settings.json now contains "addProjectBaseDirectory": "/Users/me/code/". Fixing it for one machine takes knowing to switch the settings scope to that environment first. Nothing on the default scope suggests this setting is per-machine.

Impact

Minor bug or occasional failure

Version or commit

macOS desktop 0.1.10-nightly.20261112.4564, remote Linux environment on the same version. Code references are from main @ ee90bdc.

Environment

macOS desktop app plus a remote Ubuntu server environment.

Diagnosis

This is a follow-up to #11756, which was closed after per-environment scope selection was added in #10589. Per-environment values work, but the default scope still writes one filesystem path to every machine:

• With no machine selected, the settings scope resolves to kind: "all" with every environment id (settingsScope.ts#L144-L149).
• The field commits with updateSettings({ addProjectBaseDirectory: next }) (SettingsPanels.tsx#L1751), and planScopedSettingsPatch sends it to every connected environment in that scope (scopedSettings.ts#L220-L232).
• The picker uses the stored value as-is and only falls back to ~/ when it's empty, not when the directory is missing on that machine (CommandPalette.tsx#L671-L679). AddProjectToMachineDialog.tsx#L115 does the same.

Possible fixes, any of which would help:

• Treat machine-specific path settings (addProjectBaseDirectory, and likely newProjectsRoot) as per-environment only. On the All environments scope, show the field read-only or ask which environment to set.
• Before saving a path to an environment, check it exists there, or warn when it doesn't.
• In the Add Project picker, fall back to ~/ when the configured directory doesn't exist on the target environment.

Workaround

In Settings, switch the scope selector to the remote environment and set Add project starts in there.