[Bug]: "Add project starts in" is a per-environment path exposed as a global setting, so local and remote environments cannot differ #11756

Area

apps/desktop

Steps to reproduce

1. Use T3 Code with more than one environment — for example a local macOS machine and a remote
   Linux server added as a remote environment.
2. Open Settings → General → Projects & threads and set "Add project starts in" (the
   setting stored as addProjectBaseDirectory, labelled "Add project base directory" in the
   source) to a directory that exists on one of those machines.
3. Switch to the other environment and open Add Project.

Expected behavior

The setting is a filesystem path, so it should be per-environment — each machine should be able to
have its own value, the way environmentIcon already does.

Actual behavior

There is a single value under General, which reads as an application-wide preference. A path
that is correct on one machine is meaningless on another:

macOS local machine   /Users/<user>/Code
Linux server          /srv/projects

No single string is right for both. Setting it for one environment makes it wrong for the other,
and leaving it empty falls back to ~/ — which on a headless server is often a service account's
home directory rather than anywhere projects belong.

Why this looks like an exposure bug rather than a design decision

addProjectBaseDirectory is declared in the same per-environment settings schema as
environmentIcon:

environmentIcon: Schema.optionalKey(Schema.NullOr(EnvironmentMachineKind)),
defaultThreadEnvMode: Schema.optionalKey(ThreadEnvMode),
newWorktreesStartFromOrigin: Schema.optionalKey(Schema.Boolean),
addProjectBaseDirectory: Schema.optionalKey(TrimmedString),

and the client reads it keyed by environment:

(b.find(t => t.environmentId === e)?.serverConfig?.settings ?? null)
  ?.addProjectBaseDirectory?.trim() ?? ``

So the storage and the read path are already per-environment. The difference is only where each is
surfaced in the UI:

environmentIcon           apps/web/src/components/settings/ConnectionsSettings.tsx   per-environment
addProjectBaseDirectory   apps/web/src/components/settings/SettingsPanels.tsx        General

environmentIcon can be set per environment today and is written to that environment's own
settings. This one cannot, despite being stored the same way — which suggests it is in the wrong
panel rather than intentionally global.

Suggested fix

Surface it per-environment, either by moving it beside the other per-environment settings or by
adding an environment selector to the Projects & threads group. The underlying storage appears to
need no change.

Additional: it is undocumented

addProjectBaseDirectory appears in packages/contracts/src/settings.ts, CommandPalette.tsx,
SettingsPanels.tsx, apps/mobile/.../AddProjectScreen.tsx and two test files, but the string
"base director" does not appear in any of the files under docs/user/. docs/user/source-control.md
says only "Choose a hosting provider or paste a Git URL, then choose where to save it", with no
mention that a default can be configured. Users looking for this setting cannot find it in the
manual, and at least one AI assistant invents a plausible-looking T3CODE_BASE_DIR environment
variable that does not exist in the binary.

Impact

Minor degradation or inconvenience

Version or commit

Server t3 serve 0.0.41-nightly.20260914.1707 (linux arm64), desktop app connected via T3 Connect.

Environment

macOS desktop app with a remote Linux environment over T3 Connect.