feat(web): custom stylesheets (#4602)

* add initial ui and api definitions for stylesheets

* proper saving

* make custom css work

* add textarea

* rebuild api

* run prettier

* add typecast

* update typings

* move css accordion to be sorted alphabetically

* set content-type properly

* rename stylesheets to theme

* fix server test

server/immich-openapi-specs.json

@@ -8060,6 +8060,9 @@
           "storageTemplate": {
             "$ref": "#/components/schemas/SystemConfigStorageTemplateDto"
           },
+          "theme": {
+            "$ref": "#/components/schemas/SystemConfigThemeDto"
+          },
           "thumbnail": {
             "$ref": "#/components/schemas/SystemConfigThumbnailDto"
           },
@@ -8077,7 +8080,8 @@
           "storageTemplate",
           "job",
           "thumbnail",
-          "trash"
+          "trash",
+          "theme"
         ],
         "type": "object"
       },
@@ -8404,6 +8408,17 @@
         ],
         "type": "object"
       },
+      "SystemConfigThemeDto": {
+        "properties": {
+          "customCss": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "customCss"
+        ],
+        "type": "object"
+      },
       "SystemConfigThumbnailDto": {
         "properties": {
           "colorspace": {