gui: complete Wails v2 vertical MVP — fixes, search, polished UI

Backend fixes:
- Wire search service in App struct, implement Search() bindings
- Fix OpenFile to use files.Service.Open() instead of stub
- Fix OpenFolder to open spaces/<slug>/ instead of vault root
- Remove unused imports and dead code in app.go

Frontend fixes:
- Add missing Svelte plugin to vite.config.js (blocking build error)
- Fix optional catch binding for compatibility
- Fix select dropdown rendering on Linux (appearance: none + custom arrow)
- Switch api/verstak.js to use generated Wails v2 bindings
- Include hand-written wailsjs bindings in repository
- Add build.sh to repository

Build:
  cd frontend && npm run build
  rm -rf cmd/verstak-gui/frontend-dist && cp -r frontend/dist cmd/verstak-gui/frontend-dist
  go build -tags 'gui production webkit2_41' -o verstak-gui ./cmd/verstak-gui

cmd/verstak-gui/frontend-dist/assets/index-ClxkTvdE.css

@@ -1 +0,0 @@
-.svelte-14ysusk.svelte-14ysusk,.svelte-14ysusk.svelte-14ysusk:before,.svelte-14ysusk.svelte-14ysusk:after{box-sizing:border-box;margin:0;padding:0}.app.svelte-14ysusk.svelte-14ysusk{display:flex;width:100vw;height:100vh;overflow:hidden;background:#13131f;color:#e4e4ef;font-family:-apple-system,BlinkMacSystemFont,Segoe UI,Roboto,sans-serif;font-size:14px}.sidebar.svelte-14ysusk.svelte-14ysusk{width:260px;min-width:200px;height:100vh;display:flex;flex-direction:column;background:#1a1a28;border-right:1px solid #2a2a3c;flex-shrink:0;overflow:hidden}.sidebar-top.svelte-14ysusk.svelte-14ysusk{padding:16px 20px;display:flex;align-items:center;gap:10px;border-bottom:1px solid #2a2a3c;flex-shrink:0}.logo.svelte-14ysusk.svelte-14ysusk{font-size:20px;line-height:1}.app-name.svelte-14ysusk.svelte-14ysusk{font-size:16px;font-weight:600;color:#e4e4ef}.sidebar-nav.svelte-14ysusk.svelte-14ysusk{flex:1;overflow-y:auto;padding:12px 0}.nav-group.svelte-14ysusk.svelte-14ysusk{margin-bottom:16px}.nav-label.svelte-14ysusk.svelte-14ysusk{font-size:10px;text-transform:uppercase;letter-spacing:.5px;color:#666;padding:4px 20px;margin-bottom:4px}.nav-item.svelte-14ysusk.svelte-14ysusk{display:block;width:100%;padding:8px 20px;border:none;background:none;color:#ccc;font-size:13px;text-align:left;cursor:pointer;border-radius:0;font-family:inherit}.nav-item.svelte-14ysusk.svelte-14ysusk:hover{background:#223}.nav-item.selected.svelte-14ysusk.svelte-14ysusk{background:#2a2a4a;color:#fff;font-weight:500}.nav-empty.svelte-14ysusk.svelte-14ysusk{padding:8px 20px;color:#555;font-size:12px}.sidebar-bottom.svelte-14ysusk.svelte-14ysusk{padding:12px 20px;border-top:1px solid #2a2a3c;flex-shrink:0}.version.svelte-14ysusk.svelte-14ysusk{font-size:11px;color:#555}.main.svelte-14ysusk.svelte-14ysusk{flex:1;display:flex;flex-direction:column;height:100vh;min-width:0;overflow:hidden;background:#13131f}.header.svelte-14ysusk.svelte-14ysusk{padding:12px 24px;border-bottom:1px solid #2a2a3c;display:flex;align-items:center;justify-content:space-between;flex-shrink:0;min-height:48px}.crumb.svelte-14ysusk.svelte-14ysusk{font-size:14px;font-weight:500;color:#e4e4ef}.crumb.placeholder.svelte-14ysusk.svelte-14ysusk{color:#666}.search-hint.svelte-14ysusk.svelte-14ysusk{padding:6px 12px;background:#1e1e2e;border:1px solid #2a2a3c;border-radius:4px;color:#666;font-size:12px;cursor:text}.error-banner.svelte-14ysusk.svelte-14ysusk{background:#3a2222;color:#f88;padding:8px 24px;font-size:12px;border-bottom:1px solid #4a2222;flex-shrink:0}.content.svelte-14ysusk.svelte-14ysusk{flex:1;overflow-y:auto;padding:24px}.welcome.svelte-14ysusk h2.svelte-14ysusk{font-size:28px;font-weight:300;margin-bottom:12px;color:#8888a4}.welcome.svelte-14ysusk p.svelte-14ysusk{color:#666;font-size:13px;margin-bottom:4px}.error-text.svelte-14ysusk.svelte-14ysusk{color:#f88;margin-top:12px}.loading.svelte-14ysusk.svelte-14ysusk{color:#666}.node-view.svelte-14ysusk h2.svelte-14ysusk{font-size:24px;margin-bottom:16px}.node-meta.svelte-14ysusk.svelte-14ysusk{display:flex;gap:16px;color:#666;font-size:12px}

cmd/verstak-gui/frontend-dist/index.html

@@ -16,8 +16,8 @@
       background: #13131f;
     }
   </style>
-  <script type="module" crossorigin src="/assets/index-COs6tJEl.js"></script>
-  <link rel="stylesheet" crossorigin href="/assets/index-ClxkTvdE.css">
+  <script type="module" crossorigin src="/assets/main-BqdVWy5o.js"></script>
+  <link rel="stylesheet" crossorigin href="/assets/main-D8LYjC_e.css">
 </head>
 <body>
   <div id="app"></div>