feat(housekeeping): add per-hotel theme manager with import/export and background

Theme Manager under /admin-next/hotel/theme-manager lets the owner save, apply, rename, delete, import, and export custom themes, plus set a custom site background by URL or upload. Themes are stored in WebsiteSetting/custom_themes JSON so they survive CMS updates.

src/messages/en.json

@@ -4773,6 +4773,55 @@
 					"title": "Hotel",
 					"description": "Rooms, radio, and asset tools",
 					"routes": {
+						"themeManager": {
+							"title": "Theme manager",
+							"description": "Save, switch, import, export, and publish custom hotel themes (survive updates)",
+							"currentThemeTitle": "Current theme",
+							"currentThemeHint": "The palette and background being served to your visitors right now.",
+							"saveLabel": "Save theme",
+							"savePlaceholder": "Theme name (e.g. Summer Gold)",
+							"exportCurrent": "Export",
+							"savedThemesTitle": "Saved themes",
+							"savedThemesHint": "Apply any saved theme anytime. Saved themes are stored in the database, so they keep working after CMS updates.",
+							"noThemes": "No saved themes yet — save one above.",
+							"active": "Active",
+							"apply": "Apply",
+							"export": "Export",
+							"rename": "Rename",
+							"renameSave": "OK",
+							"delete": "Delete",
+							"deleteConfirm": "Delete theme \"{name}\"?",
+							"importTitle": "Import a theme",
+							"importHint": "Upload a `.atom-theme.json` file exported here, or a theme exported from the Theme Builder.",
+							"importNameLabel": "Name",
+							"importNamePlaceholder": "Optional override",
+							"importFileLabel": "Theme file",
+							"importApplyLabel": "Apply after import",
+							"importButton": "Import",
+							"invalidFile": "That file isn't a valid JSON theme.",
+							"presetsTitle": "Quick start presets",
+							"backgroundTitle": "Background",
+							"backgroundHint": "Set a custom background image for the public site using any image URL, or upload one.",
+							"backgroundPreviewLabel": "Preview",
+							"backgroundUrlLabel": "Image URL",
+							"backgroundPositionLabel": "Position",
+							"backgroundSizeLabel": "Size",
+							"backgroundRepeatLabel": "Repeat",
+							"applyBackground": "Apply background",
+							"uploadButton": "Upload image…",
+							"clearBackground": "Remove background",
+							"notice": {
+								"saved": "Theme saved.",
+								"applied": "Theme applied.",
+								"renamed": "Theme renamed.",
+								"deleted": "Theme deleted.",
+								"imported": "Theme imported.",
+								"preset": "Preset applied.",
+								"background": "Background updated.",
+								"importError": "That file couldn't be imported. It may be too large or not a theme export.",
+								"backgroundError": "The image couldn't be uploaded. Use a PNG, JPEG, GIF, or WebP up to 10MB."
+							}
+						},
 						"nitroCleanup": {
 							"title": "Nitro cleanup",
 							"description": "Scan and remove fake, broken, and orphaned furni asset files"