Fix Nostr icon and make algo filters responsive

- Replace broken shield SVG in auth modal Nostr login button
  with a proper key icon (matches Nostr's key-based identity)
- Show algorithm filter buttons inline on xl+ screens (1280px+)
  where there's room for all of them
- Collapse into "Algos" dropdown on md–xl screens to prevent
  overflow when the header is too narrow

Co-authored-by: Cursor <cursoragent@cursor.com>

src/components/AuthModal.vue

@@ -97,8 +97,8 @@
             :disabled="isLoading"
             class="hero-info-button w-full flex items-center justify-center gap-2"
           >
-            <svg class="w-5 h-5" fill="currentColor" viewBox="0 0 24 24">
-              <path d="M12 2L2 7v10c0 5.55 3.84 10.74 9 12 5.16-1.26 9-6.45 9-12V7l-10-5z"/>
+            <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+              <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15 7a2 2 0 012 2m4 0a6 6 0 01-7.743 5.743L11 17H9v2H7v2H4a1 1 0 01-1-1v-2.586a1 1 0 01.293-.707l5.964-5.964A6 6 0 1121 9z" />
             </svg>
             Sign in with Nostr
           </button>