Add a configurable default working directory for sessions without a specific project

Problem

When starting a session without a specific project, the working directory falls back to the last selected project root, process.cwd(), or the app path — none of which the user controls. Users who don't want tasks landing in a repo root or on the system drive (e.g. C:\ on Windows) have no way to configure a catch-all directory. Real user request: "I keep a general-purpose directory (~/agent) for tasks that have no specific folder, and I want to configure that myself."

Under the hood, createProjectRootController.current() resolves as selected → persisted (lastProjectPathFile) → fallbackRoots([process.cwd(), app.getAppPath()]).

Desired outcome

• A "Default working directory" setting under Settings → General, with a system folder picker and a clear/reset action.
• Sessions created without an explicit project/folder use this directory as their cwd.
• Resolution order: explicit per-session/project selection > configured default working directory > current fallback. The setting persists across restarts and app updates.
• It also serves as the fallback cwd for bot conversations (all IM channels) without a bound folder, tying in with the "mobile session management for the bot bridge" feature (#2325).

Alternatives or workarounds

No workaround today — you must remember to pick a project each time. Reference implementations:

• Hermes Agent (desktop): resolveHermesCwd() — explicit configured dir (a JSON file in userData) > env hint > INIT_CWD > process.cwd() > repo root > home, skipping the install directory; the settings UI has a folder picker with set/clear.
• OpenClaw: agents.defaults.workspace in the global config, written by openclaw setup --workspace <path>; OPENCLAW_WORKSPACE_DIR env override; per-agent and per-session overrides.
• PawWork (negative example): the default directory is hardcoded (~/PawWork) in three places with no config entry, so tasks silently land there.