[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/io.netbird.client.plist

@@ -66,6 +66,9 @@
     <key>disableAutoConnect</key>
     <false/>
 
+    <key>disableAutostart</key>
+    <false/>
+
     <key>disableClientRoutes</key>
     <false/>
 

docs/netbird-macos.mobileconfig

@@ -103,6 +103,8 @@
                                 <!--
                                 <key>disableAutoConnect</key>
                                 <false/>
+                                <key>disableAutostart</key>
+                                <false/>
                                 <key>disableClientRoutes</key>
                                 <false/>
                                 <key>disableServerRoutes</key>

docs/netbird-macos.sh

@@ -58,6 +58,7 @@ preSharedKey="$NULL"                       # secret; redacted in log
 allowServerSSH='true'
 blockInbound="$NULL"
 disableAutoConnect="$NULL"
+disableAutostart="$NULL"
 disableClientRoutes="$NULL"
 disableServerRoutes="$NULL"
 disableMetricsCollection="$NULL"
@@ -155,6 +156,7 @@ main() {
   is_set "$allowServerSSH"            && emit_bool    allowServerSSH            "$allowServerSSH"
   is_set "$blockInbound"              && emit_bool    blockInbound              "$blockInbound"
   is_set "$disableAutoConnect"        && emit_bool    disableAutoConnect        "$disableAutoConnect"
+  is_set "$disableAutostart"          && emit_bool    disableAutostart          "$disableAutostart"
   is_set "$disableClientRoutes"       && emit_bool    disableClientRoutes       "$disableClientRoutes"
   is_set "$disableServerRoutes"       && emit_bool    disableServerRoutes       "$disableServerRoutes"
   is_set "$disableMetricsCollection"  && emit_bool    disableMetricsCollection  "$disableMetricsCollection"

docs/netbird.adml

@@ -24,6 +24,9 @@
       <string id="DisableAutoConnect_Name">Disable auto-connect</string>
       <string id="DisableAutoConnect_Help">When enabled, the NetBird tunnel does not auto-connect at daemon startup. Equivalent to --disable-auto-connect.</string>
 
+      <string id="DisableAutostart_Name">Disable autostart</string>
+      <string id="DisableAutostart_Help">When enabled, the NetBird GUI is prevented from registering itself as an OS autostart entry on fresh installs, and any existing OS autostart entry registration is removed on the next GUI launch (Windows Registry Run key, macOS Login Item, Linux .desktop). Once the admin lifts the policy, the setting stays off until the user re-enables it in Settings.</string>
+
       <string id="DisableClientRoutes_Name">Disable client routes</string>
       <string id="DisableClientRoutes_Help">When enabled, this client will not consume routes advertised by routing peers. Equivalent to --disable-client-routes.</string>
 

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)"