Disable rich presence option in MAS and Snap, and add docs link

If it won't work let's not waste the user's time.
#1006
#1005

src-main/l10n/en.json

@@ -265,7 +265,15 @@
   },
   "desktop-settings.rich-presence": {
     "string": "Enable Rich Presence",
-    "developer_comment": "Option to enable Discord rich presence support, which shows the name of the project someone is editing on their Discord profile. For compliance with Scratch policies don't use the word \"Discord\" in the translation."
+    "developer_comment": "Option to enable Discord Rich Presence support, which shows the name of the project someone is editing on their Discord profile. For compliance with Scratch policies don't use the word \"Discord\" in the translation."
+  },
+  "desktop-settings.rich-presence-unavailable": {
+    "string": "Rich Presence is not available in this version of the app",
+    "developer_comment": "Appears in desktop settings after a disabled checkbox (grayed out and unchangeable) if Discord Rich Presence can't be enabled."
+  },
+  "desktop-settings.more-information": {
+    "string": "(more information)",
+    "developer_comment": "Appears in desktop settings after options as a link to a webpage with more information about the option."
   },
   "desktop-settings.open-user-data": {
     "string": "Open User Data",

src-main/rich-presence.js

@@ -28,6 +28,7 @@ const pathUtil = require('path');
 const nodeCrypto = require('crypto');
 const {APP_NAME} = require('./brand');
 const {translate} = require('./l10n');
+const {getPlatform} = require('./platform');
 const settings = require('./settings');
 
 // Ask GarboMuffin for changes
@@ -185,12 +186,21 @@ class RichPresence {
     this.handleSocketError =  this.handleSocketError.bind(this);
   }
 
+  isAvailable () {
+    // In the Mac App Store, our tmpdir is ~/Library/Containers/org.turbowarp.desktop/Data/tmp/
+    //    while the IPC file is /var/folders/.../.../T/discord-ipc-#
+    // In the Linux Snap Store, our tmpdir is /run/user/.../snap.turbowarp-desktop/
+    //    while the IPC file is /run/user/.../snap.discord/discord-ipc-#
+    // In both cases the platform sandbox should stop us from accessing the IPC file.
+    return !(process.mas || getPlatform() === 'linux-snap');
+  }
+
   checkAutomaticEnable () {
     if (this.checkedAutomaticEnable) {
       return;
     }
     this.checkedAutomaticEnable = true;
-    if (settings.richPresence) {
+    if (settings.richPresence && this.isAvailable()) {
       this.enable();
     }
   }

src-main/windows/desktop-settings.js

@@ -34,6 +34,7 @@ class DesktopSettingsWindow extends AbstractWindow {
         bypassCORS: settings.bypassCORS,
         spellchecker: settings.spellchecker,
         exitFullscreenOnEscape: settings.exitFullscreenOnEscape,
+        richPresenceAvailable: RichPresence.isAvailable(),
         richPresence: settings.richPresence
       };
     });

src-renderer/desktop-settings/desktop-settings.html

@@ -299,14 +299,25 @@ <h1 class="title"></h1>
       <label>
         <input type="checkbox" class="rich-presence-checkbox" autocomplete="off">
         <span class="rich-presence-label"></span>
+        <a class="rich-presence-more-information" href="https://docs.turbowarp.org/desktop/rich-presence" target="_blank" rel="noreferrer"></a>
       </label>
       <script>
         const richPresence = document.querySelector('.rich-presence-checkbox');
-        richPresence.onchange = () => {
-          DesktopSettingsPreload.setRichPresence(richPresence.checked);
-        };
-        richPresence.checked = settings.richPresence;
-        document.querySelector('.rich-presence-label').textContent = strings['desktop-settings.rich-presence'];
+        const richPresenceLabel = document.querySelector('.rich-presence-label');
+        const richPresenceMoreInformation = document.querySelector('.rich-presence-more-information');
+        if (settings.richPresenceAvailable) {
+          richPresence.onchange = () => {
+            DesktopSettingsPreload.setRichPresence(richPresence.checked);
+          };
+          richPresence.checked = settings.richPresence;
+          richPresenceLabel.textContent = strings['desktop-settings.rich-presence'];
+        } else {
+          richPresence.disabled = true;
+          richPresence.checked = false;
+          richPresenceLabel.textContent = strings['desktop-settings.rich-presence-unavailable'];
+          richPresenceMoreInformation.href += '#supported-platforms';
+        }
+        richPresenceMoreInformation.textContent = strings['desktop-settings.more-information'];
       </script>
 
       <button class="open-user-data"></button>