Refactor static file path and update theme selection text in Navbar; change localization keys for login and signup buttons

frontend/src/lib/components/Navbar.svelte

@@ -222,7 +222,7 @@
 					on:click={() => (window.location.href = 'https://discord.gg/wRbQ9Egr8C')}
 					>{$t('navbar.discord')}</button
 				>
-				<p class="font-bold m-4 text-lg">{$t('navbar.theme_selection')}</p>
+				<p class="font-bold m-4 text-lg text-center">{$t('navbar.theme_selection')}</p>
 				<form method="POST" use:enhance={submitUpdateTheme}>
 					<li>
 						<button formaction="/?/setTheme&theme=light"

frontend/src/routes/+page.svelte

@@ -46,10 +46,10 @@
 						</button>
 					{:else}
 						<button on:click={() => goto('/login')} class="btn btn-primary">
-							{$t('navbar.login')}
+							{$t('auth.login')}
 						</button>
 						<button on:click={() => goto('/signup')} class="btn btn-neutral">
-							{$t('navbar.signup')}
+							{$t('auth.signup')}
 						</button>
 					{/if}
 				</div>