feat: storage path layout, basename URLs, and bidirectional domain isolation

Add storage_use_date_path for flat vs date-key storage layout and emit
basename-only public URLs when hide-folder is enabled. Block image paths
on the site host when domain separation is active so only the image host
serves files. Update settings UI, i18n, tests, and deployment docs.

i18n/locales/en.json

@@ -180,6 +180,7 @@
     "title": "System settings",
     "subtitle": "Saved to database; environment variables take precedence",
     "uploadAccount": "Upload & accounts",
+    "featureToggles": "Feature toggles",
     "defaultFolder": "Default upload folder",
     "defaultFolderHint": "Stored in this browser only; affects your upload target",
     "allowRegistration": "Open registration",
@@ -188,6 +189,8 @@
     "refererHint": "Referer hotlink protection; comma-separated hostnames, empty = no restriction",
     "refererPlaceholder": "example.com, www.example.com",
     "linkGeneration": "Domains & links",
+    "domainSettings": "Domain settings",
+    "storagePathSettings": "Storage paths & direct links",
     "domainSeparationActive": "Domain separation enabled: image host serves images only; site host serves admin and API",
     "siteBaseUrl": "Site URL",
     "siteBaseUrlHint": "No trailing slash; admin UI, API docs, and Twikoo upload endpoint",
@@ -199,10 +202,12 @@
     "imageBaseUrlPlaceholder": "https://pic.example.com",
     "imageBaseUrlActive": "Active: {url}",
     "hideFolderInUrl": "Hide folder in image URLs",
-    "hideFolderInUrlHint": "Links become /2026/08/file.webp without the images/ prefix; legacy full paths still work",
+    "hideFolderInUrlHint": "Links become /file.webp without folder or date paths; legacy full paths still work",
     "hideFolderConfirmTitle": "Hide folder prefix in URLs?",
-    "hideFolderConfirmDesc": "If two folders share the same year/month/filename (e.g. images/2026/08/a.webp and blog/2026/08/a.webp), the short URL /2026/08/a.webp resolves to the images folder first—you may get the wrong file. Enable only with a single upload folder or globally unique filenames.",
+    "hideFolderConfirmDesc": "If two folders share the same filename (e.g. images/a.webp and blog/a.webp), the short URL /a.webp resolves to the images folder first—you may get the wrong file. Enable only with a single upload folder or globally unique filenames.",
     "hideFolderConfirmEnable": "Enable anyway",
+    "storageUseDatePath": "Group uploads by year/month",
+    "storageUseDatePathHint": "When enabled, files are stored as images/2026/08/file.webp; when disabled, as images/file.webp. Applies only to new uploads after enabling",
     "badgeEnv": ".env",
     "badgeSaved": "Saved",
     "badgeDefault": "Default",