feat: complete UI redesign — editorial-paper palette, Monaco theme, lucide icons, primitives

Palette overhaul:
- Warm cream canvas (#faf8f4) + near-white surfaces (#fdfdfb) + deep teal accent (#0f766e)
- Unified on stone-warm neutrals via @theme block in index.css

Typography:
- Inter Variable + JetBrains Mono Variable via fontsource (self-hosted)
- Monaco editor set to JetBrains Mono for consistency

Icons:
- lucide-react replaces all 8 hand-rolled SVGs
- Leaf brand mark beside wordmark

Shared primitives (src/components/ui/):
- Icon, Button (4 variants, 2 sizes), IconButton, TextInput, SectionHeader, StatusDot, Chip

Custom Monaco theme (src/theme/monaco-theme.ts):
- typst-leaf-light: warm near-white background, restrained rainbow-free syntax
- Keywords teal, strings warm green, comments stone italic, most tokens ink

Component reworks:
- TopBar: Leaf icon, h-12, chip-based status, Button/IconButton primitives
- Sidebar: w-72, icon+label tabs with teal bottom border
- StatusBar: lucide GitBranch/ArrowUp/ArrowDown, chip-based status
- FileTree: lucide Plus/FolderPlus/Pencil/Trash2, teal active state
- GitPanel: lucide status icons, restored staged/unstaged bg tint
- Preview: RotateCw reload icon, chip status
- EmptyState: muted Leaf icon
- ToastHost: StatusDot + lucide X

Fixes:
- GitPanel staged/unstaged/untracked rows now have distinct bg tints
- Button defaults to type="button" to prevent accidental form submission
- Editor error fallback from red-400 to rose-600
- DESIGN.md: removed emoji from layout skeleton, fixed Modals/Monaco typo

Build: pnpm -r typecheck + pnpm build pass.

typst-leaf-frontend/src/components/ui/TextInput.tsx

@@ -0,0 +1,12 @@
+interface TextInputProps extends React.InputHTMLAttributes<HTMLInputElement> {
+  // styled input
+}
+
+export function TextInput({ className = "", ...props }: TextInputProps) {
+  return (
+    <input
+      className={`w-full px-2 py-1 text-sm bg-surface text-ink border border-line-strong rounded-md placeholder:text-ink-subtle outline-none focus:border-teal-500 focus:ring-1 focus:ring-teal-100 transition-colors duration-150 ${className}`}
+      {...props}
+    />
+  );
+}