fix: resolve sidebar nav hydration mismatch and admin events 500 error

Wrap auth-dependent sidebar navigation and meta in ClientOnly with
SSR fallback slots to prevent hydration mismatch that caused all
authenticated nav links to point to wrong pages. Fix admin events
page crash by replacing empty string USelect values with 'all'.

app/components/ColorModeToggle.vue

@@ -0,0 +1,63 @@
+<template>
+  <div class="color-mode-toggle">
+    <button
+      v-for="option in options"
+      :key="option.value"
+      :class="{ active: colorMode.preference === option.value }"
+      @click="colorMode.preference = option.value"
+    >{{ option.label }}</button>
+  </div>
+</template>
+
+<script setup>
+const colorMode = useColorMode()
+
+const options = [
+  { label: 'Light', value: 'light' },
+  { label: 'System', value: 'system' },
+  { label: 'Dark', value: 'dark' },
+]
+</script>
+
+<style scoped>
+.color-mode-toggle {
+  display: flex;
+  margin-top: 10px;
+}
+
+.color-mode-toggle button {
+  flex: 1;
+  padding: 4px 0;
+  font-family: 'Commit Mono', monospace;
+  font-size: 10px;
+  letter-spacing: 0.04em;
+  background: transparent;
+  color: var(--text-faint);
+  border: 1px dashed var(--border);
+  cursor: pointer;
+  transition: all 0.15s;
+}
+
+.color-mode-toggle button + button {
+  border-left: none;
+}
+
+.color-mode-toggle button:hover {
+  color: var(--text-dim);
+}
+
+.color-mode-toggle button.active {
+  color: var(--candle);
+  border-color: var(--candle);
+  border-style: solid;
+  background: var(--surface);
+}
+
+/* When active button is adjacent to dashed, restore left border */
+.color-mode-toggle button.active + button {
+  border-left: 1px dashed var(--border);
+}
+.color-mode-toggle button:has(+ button.active) {
+  border-right: none;
+}
+</style>