feat: add plugin UI host (sidebar, view container, settings panel) + storage API

- internal/core/storage/api.go — plugin namespace JSON storage (settings/data/cache)
- internal/core/storage/api_test.go — 8 tests (write/read, path traversal, atomic)
- internal/api/app.go — Wails bindings for storage (Read/WritePluginSettings, Read/WritePluginDataJSON)
- main.go — initialize storage service, pass to NewApp
- Sidebar.svelte — plugin sidebar items from contributions (filtered by ui.register)
- ViewContainer.svelte — plugin view host with degraded status
- PluginCard.svelte — Settings button + permission warnings
- PluginManager.svelte — settings panel modal with test form
- App.svelte — integrated sidebar + view container layout

frontend/src/lib/shell/ViewContainer.svelte

@@ -0,0 +1,137 @@
+<script>
+  import { onMount } from 'svelte';
+  import * as App from '../../../wailsjs/go/api/App';
+
+  let views = [];
+  let activeView = '';
+  let pluginStates = {};
+  let plugins = [];
+
+  onMount(async () => {
+    try {
+      const [contribs, pluginList] = await Promise.all([
+        App.GetContributions(),
+        App.GetPlugins(),
+      ]);
+      views = contribs.views || [];
+      plugins = pluginList;
+      for (const p of pluginList) {
+        pluginStates[p.manifest.id] = p.status;
+      }
+    } catch (e) {
+      console.error('[ViewContainer] load error:', e);
+    }
+
+    window.addEventListener('verstak:open-view', (e) => {
+      activeView = e.detail.viewId;
+    });
+  });
+
+  function getViewStatus(view) {
+    const status = pluginStates[view.pluginId];
+    if (status === 'failed' || status === 'incompatible') return 'error';
+    if (status === 'degraded') return 'degraded';
+    return 'ok';
+  }
+</script>
+
+<div class="view-container">
+  {#if activeView}
+    {#each views.filter(v => v.item.id === activeView) as view}
+      <div class="view" class:degraded={getViewStatus(view) === 'degraded'}>
+        <div class="view-header">
+          <span class="view-icon">{view.item.icon || '📦'}</span>
+          <h2>{view.item.title}</h2>
+          {#if getViewStatus(view) === 'degraded'}
+            <span class="badge degraded">degraded</span>
+          {/if}
+        </div>
+        <div class="view-content">
+          <div class="plugin-view-host" data-view-id={view.item.id} data-component={view.item.component}>
+            <p class="placeholder">
+              Plugin view: <strong>{view.item.component}</strong>
+              <br />
+              <span class="sub">from {view.pluginId}</span>
+            </p>
+          </div>
+        </div>
+      </div>
+    {:else}
+      <div class="empty">View "{activeView}" not found in contributions</div>
+    {/each}
+  {:else}
+    <div class="empty">
+      <p>Select an item from the sidebar</p>
+      <p class="sub">Plugin views will appear here</p>
+    </div>
+  {/if}
+</div>
+
+<style>
+  .view-container {
+    flex: 1;
+    display: flex;
+    flex-direction: column;
+    overflow: hidden;
+    background: #1a1a2e;
+  }
+  .view {
+    flex: 1;
+    display: flex;
+    flex-direction: column;
+    padding: 1.5rem;
+  }
+  .view.degraded {
+    border-left: 3px solid #ffc857;
+  }
+  .view-header {
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+    margin-bottom: 1rem;
+    padding-bottom: 0.75rem;
+    border-bottom: 1px solid #16213e;
+  }
+  .view-header h2 {
+    margin: 0;
+    font-size: 1.2rem;
+    color: #e0e0f0;
+    flex: 1;
+  }
+  .view-icon { font-size: 1.3rem; }
+  .view-content {
+    flex: 1;
+    overflow: auto;
+  }
+  .plugin-view-host {
+    width: 100%;
+    min-height: 200px;
+  }
+  .placeholder {
+    color: #666;
+    font-style: italic;
+    padding: 2rem;
+    text-align: center;
+    border: 1px dashed #333;
+    border-radius: 8px;
+  }
+  .placeholder strong { color: #4ecca3; }
+  .placeholder .sub { font-size: 0.85rem; color: #555; }
+  .empty {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    height: 100%;
+    color: #555;
+    font-size: 1rem;
+  }
+  .empty .sub { font-size: 0.85rem; color: #444; margin-top: 0.5rem; }
+  .badge {
+    padding: 0.15rem 0.5rem;
+    border-radius: 3px;
+    font-size: 0.75rem;
+    font-weight: 600;
+  }
+  .badge.degraded { background: #ffc857; color: #1a1a2e; }
+</style>