Add URL API docs to README and GitHub link to header

Document the ?data= URL parameter for programmatic invoice generation
with a collapsible example in the README. Add a "Self-host this" link
with GitHub icon to the app header bar.

Author: H1D

README.md

@@ -21,6 +21,61 @@ Inspired by [easy-invoice-pdf](https://github.com/VladSez/easy-invoice-pdf).
 - Cyrillic/Unicode support (embedded Open Sans fonts)
 - 60 e2e tests
 
+<details>
+<summary><strong>URL API — generate invoice links programmatically</strong></summary>
+
+The app accepts a `?data=` URL parameter containing a compressed invoice. You can generate these links from any language to create pre-filled invoices — useful for billing systems, CRMs, or automated workflows.
+
+**How it works:** JSON → key compression → lz-string → URL
+
+```js
+import LZString from "lz-string";
+
+// Key compression map (same keys the app uses internally)
+const KEY_MAP = {
+  language: "a", currency: "c", seller: "k", buyer: "l",
+  items: "m", name: "A", address: "B", vatNo: "C",
+  dateOfIssue: "g", dateOfService: "h", paymentDue: "r",
+  amount: "N", unit: "P", netPrice: "R", vat: "T",
+  netAmount: "V", vatAmount: "X", preTaxAmount: "Z",
+  total: "n", notes: "t",
+};
+
+function compressKeys(obj) {
+  if (Array.isArray(obj)) return obj.map(compressKeys);
+  if (obj && typeof obj === "object") {
+    return Object.fromEntries(
+      Object.entries(obj).map(([k, v]) => [KEY_MAP[k] ?? k, compressKeys(v)])
+    );
+  }
+  return obj;
+}
+
+const invoice = {
+  language: "en",
+  currency: "USD",
+  dateOfIssue: "2026-01-15",
+  dateOfService: "2026-01-15",
+  paymentDue: "2026-02-15",
+  seller: { name: "Acme Corp", address: "123 Main St" },
+  buyer:  { name: "Client Inc", address: "456 Oak Ave" },
+  items: [
+    { name: "Consulting", amount: 10, unit: "hours",
+      netPrice: 150, vat: 0, netAmount: 1500, vatAmount: 0, preTaxAmount: 1500 },
+  ],
+  total: 1500,
+};
+
+const compressed = LZString.compressToEncodedURIComponent(
+  JSON.stringify(compressKeys(invoice))
+);
+const url = `https://h1d.github.io/easypdf-lite/?data=${compressed}`;
+```
+
+Only include the fields you need — the app fills in defaults for everything else. See [`src/types.ts`](src/types.ts) for the full `InvoiceData` shape.
+
+</details>
+
 ## What you don't get
 
 API endpoints, analytics, Sentry, newsletters, about pages, or 200 MB of node_modules.

public/css/style.css

@@ -150,9 +150,33 @@ body::before {
 
 .header-right {
   display: flex;
+  align-items: center;
   gap: 8px;
 }
 
+.gh-link {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  padding: 6px 12px;
+  font-family: var(--font-body);
+  font-size: 0.78rem;
+  font-weight: 500;
+  color: var(--ink-light);
+  text-decoration: none;
+  border: 1px solid var(--border-light);
+  border-radius: var(--r-md);
+  letter-spacing: 0.01em;
+  transition: all 0.2s var(--ease-out);
+  margin-right: 4px;
+}
+
+.gh-link:hover {
+  color: var(--ink);
+  border-color: var(--border);
+  background: var(--accent-soft);
+}
+
 /* === Buttons === */
 .btn {
   display: inline-flex;
@@ -819,6 +843,8 @@ input[type="file"] {
   .header-right .btn svg + span,
   .header-right .btn:not(:has(svg)) { font-size: 0; }
   .header-right .btn { padding: 8px 12px; }
+  .gh-link span { display: none; }
+  .gh-link { padding: 6px; margin-right: 0; }
 
   .item-fields { grid-template-columns: 1fr; }
   .item-field[data-col="name"] { grid-column: span 1; }

public/index.html

@@ -16,6 +16,10 @@ <h1 class="logo">EasyPDF <span class="logo-lite">Lite</span></h1>
       <span class="tagline">Free Invoice Generator</span>
     </div>
     <div class="header-right">
+      <a href="https://github.com/h1d/easypdf-lite" target="_blank" rel="noopener" class="gh-link" data-testid="github-link">
+        <svg width="16" height="16" viewBox="0 0 16 16" fill="currentColor"><path d="M8 0C3.58 0 0 3.58 0 8c0 3.54 2.29 6.53 5.47 7.59.4.07.55-.17.55-.38 0-.19-.01-.82-.01-1.49-2.01.37-2.53-.49-2.69-.94-.09-.23-.48-.94-.82-1.13-.28-.15-.68-.52-.01-.53.63-.01 1.08.58 1.23.82.72 1.21 1.87.87 2.33.66.07-.52.28-.87.51-1.07-1.78-.2-3.64-.89-3.64-3.95 0-.87.31-1.59.82-2.15-.08-.2-.36-1.02.08-2.12 0 0 .67-.21 2.2.82.64-.18 1.32-.27 2-.27.68 0 1.36.09 2 .27 1.53-1.04 2.2-.82 2.2-.82.44 1.1.16 1.92.08 2.12.51.56.82 1.27.82 2.15 0 3.07-1.87 3.75-3.65 3.95.29.25.54.73.54 1.48 0 1.07-.01 1.93-.01 2.2 0 .21.15.46.55.38A8.01 8.01 0 0016 8c0-4.42-3.58-8-8-8z"/></svg>
+        <span>Self-host this</span>
+      </a>
       <button type="button" id="btn-share" class="btn btn-outline" data-testid="share-invoice-link-button">
         <svg width="15" height="15" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M4 12v8a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2v-8"/><polyline points="16 6 12 2 8 6"/><line x1="12" y1="2" x2="12" y2="15"/></svg>
         <span>Share Link</span>