Refactor localization strings and add missing translations

frontend/src/routes/activities/+page.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
 	import { goto } from '$app/navigation';
 	import type { PageData } from './$types';
+	import { t } from 'svelte-i18n';
 
 	export let data: PageData;
 
@@ -11,8 +12,8 @@
 <table class="table table-compact">
 	<thead>
 		<tr>
-			<th>Activity</th>
-			<th>Actions</th>
+			<th>{$t('adventures.activity')}</th>
+			<th>{$t('adventures.actions')}</th>
 		</tr>
 	</thead>
 	<tbody>
@@ -23,7 +24,7 @@
 					<button
 						class="btn btn-sm btn-primary"
 						on:click={() => goto(`/search?query=${activity}&property=activity_types`)}
-						>See Adventures</button
+						>{$t('adventures.see_adventures')}</button
 					>
 				</td>
 			</tr>