feat: add Rated At field - #4653 (#4660)

* feat(model): add Rated At field - #4653

Signed-off-by: zacaj <zacaj@zacaj.com>

* fix(ui): ignore empty dates in rating/love tooltips - #4653

* refactor(ui): add isDateSet util function

Signed-off-by: zacaj <zacaj@zacaj.com>

* feat: add tests for isDateSet and rated_at sort mappings

Added comprehensive tests for isDateSet and urlValidate functions in
ui/src/utils/validations.test.js covering falsy values, Go zero date handling,
valid date strings, Date objects, and edge cases.

Added rated_at sort mapping to album, artist, and mediafile repositories,
following the same pattern as starred_at (sorting by rating first, then by
timestamp). This enables proper sorting by rating date in the UI.

---------

Signed-off-by: zacaj <zacaj@zacaj.com>
Co-authored-by: zacaj <zacaj@zacaj.com>
Co-authored-by: Deluan <deluan@navidrome.org>

ui/src/utils/validations.js

@@ -10,3 +10,16 @@ export const urlValidate = (value) => {
     return 'ra.validation.url'
   }
 }
+
+export function isDateSet(date) {
+  if (!date) {
+    return false
+  }
+  if (typeof date === 'string') {
+    return date !== '0001-01-01T00:00:00Z'
+  }
+  if (date instanceof Date) {
+    return date.toISOString() !== '0001-01-01T00:00:00Z'
+  }
+  return !!date
+}