feat: add discord rich presence (#105)

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### Tests

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Reviewed-on: #105
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

src/lib/components/ConfigSidebar.svelte

@@ -51,6 +51,7 @@
     session_cost_budget: null,
     budget_action: "warn",
     budget_warning_threshold: 0.8,
+    discord_rpc_enabled: true,
   });
 
   let showCustomThemeEditor = $state(false);
@@ -967,6 +968,30 @@
       </div>
     </section>
 
+    <!-- Discord Rich Presence Section -->
+    <section class="pt-6 pb-6 border-t border-[var(--border-color)]">
+      <h3 class="text-lg font-semibold text-[var(--accent-primary)] mb-4 flex items-center gap-2">
+        <span>🎮</span>
+        <span>Discord Rich Presence</span>
+      </h3>
+
+      <!-- Enable/Disable Discord RPC -->
+      <div class="mb-4">
+        <label class="flex items-center gap-3 cursor-pointer">
+          <input
+            type="checkbox"
+            bind:checked={config.discord_rpc_enabled}
+            class="w-4 h-4 text-[var(--accent-primary)] bg-[var(--bg-primary)] border-[var(--border-color)] rounded focus:ring-[var(--accent-primary)] focus:ring-2"
+          />
+          <span class="text-sm text-[var(--text-primary)]">Show activity in Discord</span>
+        </label>
+      </div>
+
+      <div class="text-xs text-[var(--text-tertiary)]">
+        Display your current conversation session name and model in Discord when enabled.
+      </div>
+    </section>
+
     <!-- Save Button -->
     <div class="sticky bottom-0 pt-4 pb-2 bg-[var(--bg-secondary)]">
       <button