[auth] Allow only lower_case letter and underscore in custom icon name (#4301)

## Description

## Tests

.github/workflows/auth-lint.yml

@@ -22,6 +22,18 @@ jobs:
               with:
                   submodules: recursive
 
+            - name: Verify custom icons are lowercase including optional understores, and end with .svg
+              run: |
+                  find assets/custom-icons -type f -name "*.svg" | while read -r file; do
+                      if [[ "$(basename "$file")" != "$(basename "$file" | tr '[:upper:]' '[:lower:]' | tr ' ' '_')" ]]; then
+                          echo "File name is not lowercase: $file"
+                          exit 1
+                      fi
+                  done
+
+            - name: Verify custom icon JSON
+              run: cat assets/custom-icons/_data/custom-icons.json | jq empty
+
             - name: Install Flutter ${{ env.FLUTTER_VERSION  }}
               uses: subosito/flutter-action@v2
               with:
@@ -32,6 +44,3 @@ jobs:
             - run: flutter pub get
 
             - run: flutter analyze --no-fatal-infos
-
-            - name: Verify custom icon JSON
-              run: cat assets/custom-icons/_data/custom-icons.json | jq empty

auth/assets/custom-icons/_data/custom-icons.json

@@ -1,7 +1,8 @@
 {
   "icons": [
     {
-      "title": "1xBet"
+      "title": "1xBet",
+      "slug": "1x_bet"
     },
     {
       "title": "23andme"
@@ -45,13 +46,15 @@
     },
     {
       "title": "BaiduCloud",
+      "slug": "baidu_cloud",
       "altNames": [
         "百度云",
         "baiduyun"
       ]
     },
     {
-      "title": "Band"
+      "title": "Band",
+      "slug": "band"
     },
     {
       "title": "Battle.net",
@@ -63,6 +66,7 @@
     },
     {
       "title": "BBS.NGA",
+      "slug": "bbs_nga",
       "altNames": [
         "NGA玩家社区",
         "NGA社区"
@@ -76,6 +80,7 @@
     },
     {
       "title": "Binance",
+      "slug": "binance_exchange",
       "altNames": [
         "币安"
       ]
@@ -128,7 +133,8 @@
       ]
     },
     {
-      "title": "Bluesky"
+      "title": "Bluesky",
+      "slug": "blue_sky"
     },
     {
       "title": "Booking",
@@ -138,6 +144,7 @@
     },
     {
       "title": "BorgBase",
+      "slug": "borg_base",
       "altNames": [
         "borg"
       ]
@@ -170,7 +177,8 @@
       ]
     },
     {
-      "title": "CERN"
+      "title": "CERN",
+      "slug": "cern"
     },
     {
       "title": "ChangeNOW"
@@ -272,7 +280,8 @@
       "hex": "1DB954"
     },
     {
-      "title": "enom"
+      "title": "enom",
+      "slug": "enom_v1"
     },
     {
       "title": "Epic Games",
@@ -319,7 +328,7 @@
     },
     {
       "title": "Gosuslugi",
-      "slug": "Gosuslugi",
+      "slug": "gosuslugi",
       "altNames": [
         "Госуслуги"
       ]
@@ -366,11 +375,12 @@
       ]
     },
     {
-      "title": "IceDrive"
+      "title": "IceDrive",
+      "slug": "ice_drive"
     },
     {
       "title": "ID.me",
-      "slug": "IDme"
+      "slug": "id_me"
     },
     {
       "title": "Infomaniak"
@@ -398,7 +408,8 @@
       "hex": "000000"
     },
     {
-      "title": "IVPN"
+      "title": "IVPN",
+      "slug": "ivpn"
     },
     {
       "title": "Jagex",
@@ -428,6 +439,7 @@
     },
     {
       "title": "Ko-fi",
+      "slug": "ko_fi",
       "altNames": [
         "Ko fi",
         "Kofi"
@@ -455,6 +467,7 @@
     },
     {
       "title": "Lark",
+      "slug": "lark",
       "altNames": [
         "飞书"
       ]
@@ -464,7 +477,7 @@
     },
     {
       "title": "Linux.Do",
-      "slug": "LINUX_DO",
+      "slug": "linux_do",
       "altNames": [
         "LINUX DO",
         "LinxDo"
@@ -563,6 +576,7 @@
     },
     {
       "title": "NeteaseMail",
+      "slug": "netease_mail",
       "altNames": [
         "网易邮箱",
         "Mail.163"
@@ -632,10 +646,12 @@
       "hex": "f08222"
     },
     {
-      "title": "pCloud"
+      "title": "pCloud",
+      "slug": "pcloud"
     },
     {
       "title": "PebbleHost",
+      "slug": "pebble_host",
       "altNames": [
         "Pebble Host"
       ]
@@ -712,6 +728,7 @@
     },
     {
       "title": "Restream",
+      "slug": "restream",
       "altNames": [
         "restream.io"
       ]
@@ -834,6 +851,7 @@
     },
     {
       "title": "Terabit",
+      "slug": "terabit",
       "altNames": [
         "Terabit Hosting",
         "terabit.io"
@@ -962,6 +980,7 @@
     },
     {
       "title": "Yandex",
+      "slug": "yandex",
       "altNames": [
         "Ya",
         "Яндекс"