[client] checks/enforce MDM disableAutostart on every GUI launch, not just fresh installs (#6782)

## Describe your changes

`applyAutostartDefault` gated all MDM enforcement behind the one-time
`AutostartInitialized` marker, so MDM `disableAutostart` only affected
fresh installs — a policy pushed after autostart had been enabled could
not
revoke the OS login-item. 

The PR adds follow-up MDM enforcements at the top of
the function: if at any time MDM sets `disableAutostart=true` and the OS
registration is
present, force `SetEnabled(false)` to align it.

Trade-off: once the admin lifts the policy, autostart stays off until
the user re-toggles
in Settings — consistent with "MDM always wins" behavior of the other
managed keys.

## Issue ticket number and link

Follow-up to PR https://github.com/netbirdio/netbird/pull/6738
(introduced
the `disableAutostart` MDM key with fresh-install-only semantics).

## Stack

<!-- branch-stack -->

### Checklist
- [x] Is it a bug fix
- [ ] Is a typo/documentation fix
- [ ] Is a feature enhancement
- [ ] It is a refactor
- [ ] Created tests that fail without the change (if possible)

> By submitting this pull request, you confirm that you have read and
agree to the terms of the [Contributor License
Agreement](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTOR_LICENSE_AGREEMENT.md).

## Documentation
Select exactly one:

- [x] I added/updated documentation for this change
- [ ] Documentation is **not needed** for this change (explain why)

### Docs PR URL (required if "docs added" is checked)
Paste the PR link from https://github.com/netbirdio/docs here:

<https://github.com/netbirdio/docs/pull/855>


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Added an administrative policy to disable client autostart (“Disable
Autostart”).
- Added support for configuring this policy via macOS MDM, Windows Group
Policy (ADMX/ADML), and registry settings.
- When enforced, the client prevents new autostart registration on fresh
installs and removes existing autostart on the next GUI launch, keeping
it disabled until the policy is lifted.

- **Bug Fixes**
- Improved enforcement logic for managed autostart defaults so policy
state is applied consistently during startup and first-run setup.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

docs/netbird.admx

@@ -64,6 +64,18 @@
       <disabledValue><decimal value="0" /></disabledValue>
     </policy>
 
+    <policy name="DisableAutostart"
+            class="Machine"
+            displayName="$(string.DisableAutostart_Name)"
+            explainText="$(string.DisableAutostart_Help)"
+            key="Software\Policies\NetBird"
+            valueName="DisableAutostart">
+      <parentCategory ref="NetBird" />
+      <supportedOn ref="SUPPORTED_NetBird_All" />
+      <enabledValue><decimal value="1" /></enabledValue>
+      <disabledValue><decimal value="0" /></disabledValue>
+    </policy>
+
     <policy name="DisableClientRoutes"
             class="Machine"
             displayName="$(string.DisableClientRoutes_Name)"