feat: dark-tech UI redesign + markdown patch cards

Redesign the frontend with a dark-tech theme: add Tailwind + PostCSS,
design tokens, and shadcn-style primitives (Button/Card/Input/Dialog/
DropdownMenu/Tabs/ScrollArea/etc.); restyle the app shell, navigation,
sidebar tree, content view, markdown rendering, editors, modals and
settings panels. Behavior/props unchanged; Font Awesome replaced with
lucide-react.

Add the patch cards feature UI: patch-queries hooks and a PatchCards
component rendered below the markdown body, with an Add Patch button
and create/edit dialog.

Fix tree expandability: folders with an index page now expand on name
click (and navigate), and the chevron+folder icon is one larger toggle.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/PathManager.css

@@ -1,39 +1,5 @@
-.path-manager-body {
-    display: flex;
-    flex-direction: column;
-    gap: 0.5rem;
-}
-
-.dropdown {
-    display: flex;
-    align-items: center;
-    position: relative;
-}
-
-.dropdown .dropdown-input {
-    flex: 1;
-}
-
-.dropdown .dropdown-menu {
-    position: absolute;
-    top: 100%;
-    left: 0;
-    width: 100%;
-    z-index: 10;
-    background: white;
-    border: 1px solid #ddd;
-    border-radius: 0.25rem;
-    box-shadow: 0 2px 5px rgba(0, 0, 0, 0.1);
+/* Styling moved to Tailwind / dark-tech design system in PathManager.js.
+   Only the scroll cap for the suggestions dropdown remains here. */
+.path-manager-dropdown {
     max-height: 200px;
-    overflow-y: auto;
-}
-
-.dropdown-item {
-    padding: 0.5rem;
-    cursor: pointer;
-    transition: background-color 0.2s;
-}
-
-.dropdown-item:hover {
-    background-color: #f0f0f0;
 }