feat: implement tenant OAuth flow and guest achievements

docs/wireframes/guest-pwa.md

@@ -12,13 +12,15 @@ flowchart TD
   E --> F[Upload Queue]
   D --> G[Photo Lightbox]
   D --> H[Filters]
-  D --> I[Settings]
+  D --> I[Settings Sheet]
   F --> D
   G --> D
 ```
 
 Gallery (mobile)
 
+Settings open in-place via a sheet launched from the header gear; the sheet focuses on legal text and cache management while the theme toggle remains in the header.
+
 ```
 +--------------------------------------------------+
 |  Toolbar:  [Scan?]  [Filter]   [Upload]  [Menu]  |