Harden OOBE, launch-source and elevation flow

Introduce a per-user OOBE state model and hardened launch/elevation handling. Adds OobeStateFile/OobeLaunchDecision models, OobeStateService (persisting %LOCALAPPDATA%/.launcher/state/oobe-state.json), and LauncherExecutionContext to capture elevation and user SID. CommandContext now normalizes/infers launch-source values (normal, postinstall, apply-update, plugin-install, debug-preview) and exposes maintenance checks. LauncherFlowCoordinator propagates richer launcher context details for diagnostics and suppresses OOBE for elevated/maintenance contexts. PluginInstallerService avoids requesting elevation for user-scoped installs and returns a clear error when installation target is outside the current user's LocalAppData. LauncherClient maps and surfaces result codes, UpdateWorkflow and installer invocation now pass explicit --launch-source values, and WelcomeOobeStep persists OOBE completion via the new service. Adds unit tests (CommandContext, OobeStateService, PluginInstallerService), docs/specs/checklists for the contract, and makes internals visible to tests.

.trae/specs/launcher-oobe-elevation-hardening/spec.md

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+# Launcher OOBE and Elevation Hardening Spec
+
+## Goal
+
+Stabilize the launcher startup path so that:
+
+- OOBE does not reappear for the same Windows user after reinstall/upgrade.
+- Normal startup, OOBE, update checks, incremental downloads, and default plugin installs do not trigger unexpected UAC prompts.
+- Only the approved elevation paths remain allowed.
+
+## Scope
+
+- Launcher OOBE state handling
+- launch source classification
+- elevation boundary cleanup
+- plugin install default behavior
+- diagnostic logging and troubleshooting guidance
+
+## Behavior
+
+- OOBE state is stored as a per-user truth source at `%LOCALAPPDATA%\LanMountainDesktop\.launcher\state\oobe-state.json`.
+- `first_run_completed` is treated as a legacy compatibility marker only.
+- `launchSource` values are treated as:
+  - `normal`
+  - `postinstall`
+  - `apply-update`
+  - `plugin-install`
+  - `debug-preview`
+- Automatic OOBE is allowed only for normal user-mode startup.
+- `postinstall` may show OOBE only when the launcher is not elevated and user state is available.
+- `apply-update`, `plugin-install`, and `debug-preview` must not auto-enter OOBE.
+- Allowed elevation paths are limited to:
+  - the installer itself
+  - full installer update application
+  - user-confirmed legacy uninstall
+- Default plugin installation targets the current user's LocalAppData scope and must not request elevation by default.
+
+## Acceptance
+
+- Same-user reinstall does not re-enter OOBE.
+- Missing or damaged OOBE state does not silently bounce the user back into OOBE loops.
+- Default plugin installation path never triggers surprise UAC.
+- Logs can explain why OOBE was shown or suppressed and why elevation was or was not requested.