Redesign header UI with dropdown menu for vault filter and theme toggle

frontend/style.css

@@ -157,19 +157,11 @@ a:hover {
   pointer-events: none;
 }
 
-.search-vault-filter {
-  padding: 6px 10px;
-  border: 1px solid var(--border);
-  border-radius: 8px;
-  background: var(--search-bg);
-  color: var(--text-primary);
-  font-family: 'JetBrains Mono', monospace;
-  font-size: 0.8rem;
-  outline: none;
-  cursor: pointer;
+.header-menu {
+  position: relative;
 }
 
-.theme-toggle {
+.header-menu-btn {
   background: none;
   border: 1px solid var(--border);
   border-radius: 8px;
@@ -180,10 +172,90 @@ a:hover {
   align-items: center;
   transition: color 200ms ease, border-color 200ms ease;
 }
-.theme-toggle:hover {
+.header-menu-btn:hover {
   color: var(--accent);
   border-color: var(--accent);
 }
+.header-menu-btn.active {
+  color: var(--accent);
+  border-color: var(--accent);
+  background: var(--tag-bg);
+}
+
+.header-menu-dropdown {
+  display: none;
+  position: absolute;
+  top: calc(100% + 8px);
+  right: 0;
+  background: var(--bg-secondary);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  box-shadow: 0 4px 16px rgba(0,0,0,0.3);
+  min-width: 220px;
+  z-index: 100;
+  padding: 8px;
+}
+.header-menu-dropdown.active {
+  display: block;
+}
+
+.menu-section {
+  padding: 8px;
+}
+
+.menu-label {
+  font-family: 'JetBrains Mono', monospace;
+  font-size: 0.7rem;
+  color: var(--text-muted);
+  text-transform: uppercase;
+  letter-spacing: 0.5px;
+  margin-bottom: 6px;
+  font-weight: 600;
+}
+
+.menu-select {
+  width: 100%;
+  padding: 6px 10px;
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  background: var(--search-bg);
+  color: var(--text-primary);
+  font-family: 'JetBrains Mono', monospace;
+  font-size: 0.8rem;
+  outline: none;
+  cursor: pointer;
+  transition: border-color 200ms ease;
+}
+.menu-select:hover, .menu-select:focus {
+  border-color: var(--accent);
+}
+
+.menu-divider {
+  height: 1px;
+  background: var(--border);
+  margin: 4px 0;
+}
+
+.menu-theme-btn {
+  width: 100%;
+  background: none;
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  padding: 8px 12px;
+  cursor: pointer;
+  color: var(--text-primary);
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  font-family: 'JetBrains Mono', monospace;
+  font-size: 0.8rem;
+  transition: all 200ms ease;
+}
+.menu-theme-btn:hover {
+  background: var(--bg-hover);
+  border-color: var(--accent);
+  color: var(--accent);
+}
 
 /* --- Main body --- */
 .main-body {
@@ -938,9 +1010,13 @@ body.resizing-v {
     font-size: 0.8rem;
   }
 
-  .search-vault-filter {
-    font-size: 0.7rem;
-    padding: 4px 6px;
+  .header-menu-btn {
+    padding: 5px 8px;
+  }
+
+  .header-menu-dropdown {
+    right: -8px;
+    min-width: 200px;
   }
 
   .sidebar {