Phase 1 MVP - Complete implementation

- Login with JWT and refresh token rotation
- Dashboard with projects cards
- ProjectView with TreeView navigation
- DocumentView with markdown editor and auto-save
- Tag management (create, assign, remove)
- Dark mode CSS variables
- Security fixes applied (logout to backend, createDocument endpoint)

node_modules/postcss/lib/at-rule.js

@@ -0,0 +1,25 @@
+'use strict'
+
+let Container = require('./container')
+
+class AtRule extends Container {
+  constructor(defaults) {
+    super(defaults)
+    this.type = 'atrule'
+  }
+
+  append(...children) {
+    if (!this.proxyOf.nodes) this.nodes = []
+    return super.append(...children)
+  }
+
+  prepend(...children) {
+    if (!this.proxyOf.nodes) this.nodes = []
+    return super.prepend(...children)
+  }
+}
+
+module.exports = AtRule
+AtRule.default = AtRule
+
+Container.registerAtRule(AtRule)