feat(ui): Add customizable OIDC login screen branding (#1583)

* feat(ui): Add customizable OIDC login screen branding

As per https://github.com/TwiN/gatus/discussions/1579, this PR
allows for some customizations in the OIDC login screen:

- If a logo is set it will be displayed alongside the Gatus one
- New ui config `ui.login-subtitle`: customises the message in
  the screen
- If set, use `ui.header` will be used to customise the title

I haven't commited the web static assets. I believe that's
triggered with a comment? let me know if I should be doing it.

Also please clarify if I've forgotten about something here,
this is my first pr.

* chore(ui): Regenerate static assets

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: TwiN <twin@linux.com>

web/app/src/App.vue

@@ -112,13 +112,14 @@
     <div v-else id="login-container" class="flex items-center justify-center min-h-screen p-4">
       <Card class="w-full max-w-md">
         <CardHeader class="text-center">
-          <img 
-            src="./assets/logo.svg" 
-            alt="Gatus" 
-            class="w-20 h-20 mx-auto mb-4"
-          />
-          <CardTitle class="text-3xl">Gatus</CardTitle>
-          <p class="text-muted-foreground mt-2">System Monitoring Dashboard</p>
+          <div v-if="logo" class="flex items-center justify-center gap-4 mb-4">
+            <img :src="logo" alt="" class="w-20 h-20 object-contain" />
+            <div class="w-px h-12 bg-border"></div>
+            <img src="./assets/logo.svg" alt="Gatus" class="w-20 h-20" />
+          </div>
+          <img v-else src="./assets/logo.svg" alt="Gatus" class="w-20 h-20 mx-auto mb-4" />
+          <CardTitle class="text-3xl">{{ header }}</CardTitle>
+          <p class="text-muted-foreground mt-2">{{ loginSubtitle }}</p>
         </CardHeader>
         <CardContent>
           <div v-if="route && route.query.error" class="mb-6">
@@ -191,6 +192,10 @@ const buttons = computed(() => {
   return window.config && window.config.buttons ? window.config.buttons : []
 })
 
+const loginSubtitle = computed(() => {
+  return window.config && window.config.loginSubtitle && window.config.loginSubtitle !== '{{ .UI.LoginSubtitle }}' ? window.config.loginSubtitle : "System Monitoring Dashboard"
+})
+
 // Methods
 const fetchConfig = async () => {
   try {