feat(profile): keep_screen_awake user preference (default on)

New users column surfaced as an On/Off dropdown in the profile form.
The form and API carry "true"/"false" strings with the theme-style
""-guard, so JSON clients that omit the field can't clobber it;
anything else is rejected. GET/PUT /api/auth/profile expose
keep_screen_awake alongside the other profile fields, and the reader
route threads the preference into its template metadata.

Integration tests pin the schema default, both values, omission
semantics, and rejection — dedicated test user, never testuser.

internal/router/reader.go

@@ -100,6 +100,7 @@ func registerReaderRoutes(cfg *Config) {
 			TotalCharacters:  mediaItem.TotalCharacters.Int64,
 			EstimatedPages:   sync.EstimatedPages(mediaItem.TotalCharacters.Int64),
 			TreatAsComic:     handlers.ShouldTreatAsComic(libraryWithType.TypeName, mediaItem.FormatGroup),
+			KeepScreenAwake:  user.KeepScreenAwake,
 		}
 		// Render template
 		var buf bytes.Buffer