feat(reader): add reader JavaScript loading and backend support

- templates/reader.templ: add script tag to load main.js
- Enables reader shell and Alpine components to initialize
- internal/handlers/reader.go: add GetMediaReadingProgress handler
- Backend now supports fetching reading progress via API
- GET /api/media-items/:id/progress returns current progress

templates/reader.templ

@@ -12,6 +12,7 @@ templ Reader(user User, metadata ReaderMetadata, progress ReadingProgress, bookm
 			<link rel="manifest" href="/static/manifest.json"/>
 			<link href="/static/reader-fonts.css" rel="stylesheet"/>
 			<script src="/static/htmx.min.js"></script>
+			<script src="/static/main.js" defer></script>
 			<link href="/static/style.css" rel="stylesheet"/>
 		</head>
 		<body