feat: add model selector, skills/memory pages, and config management

- Add model selector in sidebar that discovers models from auth.json credential pool
- Add per-session model display (badge in chat header and session list)
- Add skills browser page and memory editor page
- Add BFF routes for skills, memory, and config model management
- Model switching updates config.yaml provider field to bypass env auto-detection
- Refactor Settings page, simplify ChatInput with file upload
- Add attachment upload support via BFF /upload endpoint

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/layout/ModelSelector.vue

@@ -0,0 +1,55 @@
+<script setup lang="ts">
+import { computed } from 'vue'
+import { NSelect } from 'naive-ui'
+import { useAppStore } from '@/stores/app'
+
+const appStore = useAppStore()
+
+const options = computed(() =>
+  appStore.modelGroups.map(g => ({
+    label: g.label,
+    type: 'group' as const,
+    key: g.provider,
+    children: g.models.map(m => ({
+      label: m,
+      value: m,
+    })),
+  })),
+)
+
+function handleChange(value: string | number | Array<string | number>) {
+  if (typeof value === 'string') {
+    appStore.switchModel(value)
+  }
+}
+</script>
+
+<template>
+  <div class="model-selector">
+    <div class="model-label">Model</div>
+    <NSelect
+      :value="appStore.selectedModel"
+      :options="options"
+      size="small"
+      @update:value="handleChange"
+    />
+  </div>
+</template>
+
+<style scoped lang="scss">
+@use '@/styles/variables' as *;
+
+.model-selector {
+  padding: 0 12px;
+  margin-bottom: 8px;
+}
+
+.model-label {
+  font-size: 11px;
+  font-weight: 600;
+  color: $text-muted;
+  text-transform: uppercase;
+  letter-spacing: 0.5px;
+  margin-bottom: 6px;
+}
+</style>