feat(ui): redesign UI with Tailwind CSS, CodeMirror 6, and new layout

- Replace all hand-written CSS with Tailwind utility classes
- Upgrade CodeMirror 5 to CodeMirror 6 (scoped @codemirror/* packages)
- Redesign layout with responsive sidebar navigation using JSX/Mithril
- Flatten admin routes (e.g. /admin/presets -> /presets)
- Add "remember me" login option with 2-week persistent cookie
- Add delete confirmation dialogs across admin pages
- Upgrade esbuild from 0.21 to 0.27 and update browser targets
- Add Inter and Roboto Mono variable fonts with subset script

ui/codemirror-loader.ts

@@ -0,0 +1,9 @@
+export { EditorView, keymap, lineNumbers } from "@codemirror/view";
+export { EditorState } from "@codemirror/state";
+export { history, historyKeymap, defaultKeymap } from "@codemirror/commands";
+export {
+  syntaxHighlighting,
+  defaultHighlightStyle,
+} from "@codemirror/language";
+export { javascript } from "@codemirror/lang-javascript";
+export { yaml } from "@codemirror/lang-yaml";