Add new styles for app shell, tokens, and vault components

- Introduced `shell.css` for the main application layout, including styles for the app shell, top bar, and user interactions.
- Created `tokens.css` to define CSS variables for theming, including colors, shadows, and transition durations for light and dark modes.
- Developed `vault.css` for the vault component, implementing grid layouts, sidebar styles, search inputs, and list item designs.

webapp/src/styles/base.css

@@ -0,0 +1,27 @@
+* {
+  box-sizing: border-box;
+}
+
+html,
+body,
+#root {
+  margin: 0;
+  padding: 0;
+  width: 100%;
+  height: 100%;
+  color: var(--text);
+  background: var(--bg-accent);
+  font-family: 'Segoe UI', 'PingFang SC', 'Microsoft YaHei', 'Noto Sans SC', sans-serif;
+}
+
+body {
+  position: relative;
+  transition:
+    background-color var(--dur-medium) var(--ease-smooth),
+    color var(--dur-medium) var(--ease-smooth);
+}
+
+body.dialog-open {
+  overflow: hidden;
+  overscroll-behavior: contain;
+}