refactor: split project into web and server directories

- Move frontend to web/ directory
- Add server/ directory for backend
- Restructure project for前后端分离架构

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/src/assets/styles/app.css

@@ -0,0 +1,49 @@
+.app {
+  min-height: 100dvh;
+  display: grid;
+  grid-template-columns: 220px minmax(0, 1fr);
+  background: var(--bg);
+}
+.main { min-width: 0; display: grid; grid-template-rows: auto auto minmax(0, 1fr); }
+.main.overview-main {
+  grid-template-rows: auto minmax(0, 1fr);
+}
+.main.workbench-main {
+  grid-template-rows: auto minmax(0, 1fr);
+}
+.main.chat-main {
+  height: 100dvh;
+  grid-template-rows: auto minmax(0, 1fr);
+  overflow: hidden;
+}
+.main.requests-main,
+.main.approval-main,
+.main.policies-main,
+.main.audit-main,
+.main.employees-main {
+  height: 100dvh;
+  grid-template-rows: auto minmax(0, 1fr);
+  overflow: hidden;
+}
+.workarea { overflow: auto; padding: 24px; }
+.workarea.chat-workarea {
+  min-height: 0;
+  overflow: hidden;
+}
+.workarea.requests-workarea,
+.workarea.approval-workarea,
+.workarea.policies-workarea,
+.workarea.audit-workarea,
+.workarea.employees-workarea {
+  min-height: 0;
+  overflow: hidden;
+  padding: 20px 24px;
+}
+
+@media (max-width: 1180px) {
+  .app { grid-template-columns: 220px minmax(0, 1fr); }
+}
+@media (max-width: 760px) {
+  .app { display: block; }
+  .workarea { padding: 18px 16px 28px; }
+}