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/App.css

@@ -1,21 +1,4 @@
-.app-container {
-    display: flex;
-    flex-direction: column;
-    height: 100vh;
-}
-
-.content-container {
-    display: flex;
-    flex: 1;
-    overflow: hidden;
-}
-
-.main-content {
-    flex: 1;
-    padding: 1rem 1rem 100px 1rem;
-    overflow-y: auto;
-}
-
+/* Layout now lives in App.js via Tailwind; keep body from double-scrolling. */
 :root {
     overflow: hidden;
-}
+}