ipn/ipnlocal,util/syspolicy,docs/windows/policy: implement the ReconnectAfter policy setting

In this PR, we update the LocalBackend so that when the ReconnectAfter policy setting is configured
and a user disconnects Tailscale by setting WantRunning to false in the profile prefs, the LocalBackend
will now start a timer to set WantRunning back to true once the ReconnectAfter timer expires.

We also update the ADMX/ADML policy definitions to allow configuring this policy setting for Windows
via Group Policy and Intune.

Updates #14824

Signed-off-by: Nick Khyl <nickk@tailscale.com>

docs/windows/policy/en-US/tailscale.adml

@@ -109,6 +109,14 @@ If you enable this policy setting, users will not be allowed to disconnect Tails
 If necessary, it can be used along with Unattended Mode to keep Tailscale connected regardless of whether a user is logged in. This can be used to facilitate remote access to a device or ensure connectivity to a Domain Controller before a user logs in.
 
 If you disable or don't configure this policy setting, users will be allowed to disconnect Tailscale at their will.]]></string>
+            <string id="ReconnectAfter">Configure automatic reconnect delay</string>
+            <string id="ReconnectAfter_Help"><![CDATA[This policy setting controls when Tailscale will attempt to reconnect automatically after a user disconnects it. It helps users remain connected most of the time and retain access to corporate resources without preventing them from temporarily disconnecting Tailscale. To configure whether and when Tailscale can be disconnected, see the "Restrict users from disconnecting Tailscale (always-on mode)" policy setting.
+
+If you enable this policy setting, you can specify how long Tailscale will wait before attempting to reconnect after a user disconnects. The value should be specified as a Go duration: for example, 30s, 5m, or 1h30m. If the value is left blank, or if the specified duration is zero, Tailscale will not attempt to reconnect automatically.
+
+If you disable or don't configure this policy setting, Tailscale will only reconnect if a user chooses to or if required by a different policy setting.
+
+Refer to https://pkg.go.dev/time#ParseDuration for information about the supported duration strings.]]></string>
             <string id="ExitNodeAllowLANAccess">Allow Local Network Access when an Exit Node is in use</string>
             <string id="ExitNodeAllowLANAccess_Help"><![CDATA[This policy can be used to require that the Allow Local Network Access setting is configured a certain way.
 
@@ -280,6 +288,12 @@ See https://tailscale.com/kb/1315/mdm-keys#set-your-organization-name for more d
                 <text>The options below allow configuring exceptions where disconnecting Tailscale is permitted.</text>
                 <dropdownList refId="AlwaysOn_OverrideWithReason" noSort="true" defaultItem="0">Disconnects with reason:</dropdownList>
             </presentation>
+            <presentation id="ReconnectAfter">
+                <text>The delay must be a valid Go duration string, such as 30s, 5m, or 1h30m, all without spaces or any other symbols.</text>
+                <textBox refId="ReconnectAfterDelay">
+                    <label>Reconnect after:</label>
+                </textBox>
+            </presentation>
             <presentation id="ExitNodeID">
                 <textBox refId="ExitNodeIDPrompt">
                     <label>Exit Node:</label>

docs/windows/policy/tailscale.admx

@@ -156,6 +156,13 @@
         </enum>
       </elements>
     </policy>
+    <policy name="ReconnectAfter" class="Machine" displayName="$(string.ReconnectAfter)" explainText="$(string.ReconnectAfter_Help)" presentation="$(presentation.ReconnectAfter)" key="Software\Policies\Tailscale">
+      <parentCategory ref="Settings_Category" />
+      <supportedOn ref="SINCE_V1_82" />
+      <elements>
+        <text id="ReconnectAfterDelay" valueName="ReconnectAfter" required="true" />
+      </elements>
+    </policy>
     <policy name="ExitNodeAllowLANAccess" class="Machine" displayName="$(string.ExitNodeAllowLANAccess)" explainText="$(string.ExitNodeAllowLANAccess_Help)" key="Software\Policies\Tailscale" valueName="ExitNodeAllowLANAccess">
       <parentCategory ref="Settings_Category" />
       <supportedOn ref="PARTIAL_FULL_SINCE_V1_56" />