[web] Fix missing translations in placeholder

Placeholders for input fields are now translated.

web-src/src/pages/SettingsPageOnlineServices.vue

@@ -94,7 +94,7 @@
                     v-model="lastfm_login.user"
                     class="input"
                     type="text"
-                    placeholder="Username"
+                    :placeholder="$t('page.settings.services.username')"
                   />
                   <p class="help is-danger" v-text="lastfm_login.errors.user" />
                 </div>
@@ -103,7 +103,7 @@
                     v-model="lastfm_login.password"
                     class="input"
                     type="password"
-                    placeholder="Password"
+                    :placeholder="$t('page.settings.services.password')"
                   />
                   <p
                     class="help is-danger"

web-src/src/pages/SettingsPageRemotesOutputs.vue

@@ -19,7 +19,7 @@
                   v-model="pairing_req.pin"
                   class="input"
                   type="text"
-                  placeholder="Enter pairing code"
+                  :placeholder="$t('page.settings.devices.pairing-code')"
                 />
               </div>
               <div class="control">

web-src/src/pages/SettingsPageWebinterface.vue

@@ -124,7 +124,7 @@
           category_name="webinterface"
           option_name="show_composer_for_genre"
           :disabled="!settings_option_show_composer_now_playing"
-          placeholder="Genres"
+          :placeholder="$t('page.settings.general.genres')"
         >
           <template #label>
             <span v-text="$t('page.settings.general.show-composer-genres')" />