diff --git a/public/js/modalFocus.js b/public/js/modalFocus.js
new file mode 100644
index 0000000..813a7f0
--- /dev/null
+++ b/public/js/modalFocus.js
@@ -0,0 +1,40 @@
+const focusableSelector = 'a[href], button:not([disabled]), input:not([disabled]), select:not([disabled]), textarea:not([disabled]), [tabindex]:not([tabindex="-1"])';
+const stack = [];
+
+function focusables(modal) {
+    return [...modal.querySelectorAll(focusableSelector)].filter(element =>
+        element.getClientRects().length && !element.closest('.hidden, [hidden]'));
+}
+
+export function openModal(modal, preferredFocus) {
+    const previous = document.activeElement;
+    modal.classList.remove('hidden');
+    stack.push({modal, previous});
+    (preferredFocus || focusables(modal)[0] || modal.querySelector('[role="dialog"]'))?.focus();
+}
+
+export function closeModal(modal) {
+    const index = stack.findLastIndex(entry => entry.modal === modal);
+    if (index < 0) return;
+    const [{previous}] = stack.splice(index, 1);
+    modal.classList.add('hidden');
+    if (index === stack.length) {
+        const target = stack.length ? focusables(stack.at(-1).modal)[0] : previous;
+        if (target?.isConnected) target.focus();
+    }
+}
+
+export function topModal() { return stack.at(-1)?.modal || null; }
+
+document.addEventListener('keydown', event => {
+    if (event.key !== 'Tab' || !stack.length) return;
+    const modal = topModal();
+    const elements = focusables(modal);
+    if (!elements.length) { event.preventDefault(); modal.querySelector('[role="dialog"]')?.focus(); return; }
+    const first = elements[0], last = elements.at(-1);
+    if (event.shiftKey && (document.activeElement === first || !modal.contains(document.activeElement))) {
+        event.preventDefault(); last.focus();
+    } else if (!event.shiftKey && (document.activeElement === last || !modal.contains(document.activeElement))) {
+        event.preventDefault(); first.focus();
+    }
+}, true);
diff --git a/src/services/workspaceArchive.js b/src/services/workspaceArchive.js
new file mode 100644
index 0000000..4c12099
--- /dev/null
+++ b/src/services/workspaceArchive.js
@@ -0,0 +1,61 @@
+/** Portable workspace archive: a gzip-compressed ustar file with JSON and images. */
+import {gzipSync, gunzipSync} from "node:zlib";
+
+export const MAX_ARCHIVE_BYTES = 250 * 1024 * 1024;
+
+function header(name, size) {
+    const block = Buffer.alloc(512);
+    block.write(name, 0, 100, "ascii");
+    block.write("0000644\0", 100, "ascii");
+    block.write("0000000\0", 108, "ascii");
+    block.write("0000000\0", 116, "ascii");
+    block.write(size.toString(8).padStart(11, "0") + "\0", 124, "ascii");
+    block.write(Math.floor(Date.now() / 1000).toString(8).padStart(11, "0") + "\0", 136, "ascii");
+    block.fill(32, 148, 156);
+    block.write("0", 156, "ascii");
+    block.write("ustar\0", 257, "ascii");
+    block.write("00", 263, "ascii");
+    const checksum = block.reduce((sum, byte) => sum + byte, 0);
+    block.write(checksum.toString(8).padStart(6, "0") + "\0 ", 148, "ascii");
+    return block;
+}
+
+export function createWorkspaceArchive(workspace, images) {
+    const files = [["workspace.json", Buffer.from(JSON.stringify(workspace, null, 2))], ...images];
+    const chunks = [];
+    let total = 1024;
+    for (const [name, body] of files) {
+        if (!/^(workspace\.json|images\/[a-f0-9-]{36}\.(png|jpeg|webp))$/.test(name)) throw new Error("Invalid archive entry");
+        total += 512 + Math.ceil(body.length / 512) * 512;
+        if (total > MAX_ARCHIVE_BYTES) throw new Error("Workspace archive exceeds 250 MB");
+        chunks.push(header(name, body.length), body, Buffer.alloc((512 - body.length % 512) % 512));
+    }
+    chunks.push(Buffer.alloc(1024));
+    return gzipSync(Buffer.concat(chunks, total), {level: 1});
+}
+
+export function parseWorkspaceArchive(buffer) {
+    if (!Buffer.isBuffer(buffer) || buffer.length > MAX_ARCHIVE_BYTES) throw new Error("Archive exceeds 250 MB");
+    const tar = gunzipSync(buffer, {maxOutputLength: MAX_ARCHIVE_BYTES});
+    const files = new Map();
+    for (let offset = 0; offset + 512 <= tar.length;) {
+        const block = tar.subarray(offset, offset + 512);
+        if (block.every(byte => byte === 0)) break;
+        const name = block.toString("ascii", 0, 100).split("\0")[0];
+        if (!/^(workspace\.json|images\/[a-f0-9-]{36}\.(png|jpeg|webp))$/.test(name) || files.has(name) || block.toString("ascii", 156, 157) !== "0") {
+            throw new Error("Invalid archive entry");
+        }
+        const recorded = Number.parseInt(block.toString("ascii", 148, 156).trim(), 8);
+        const copy = Buffer.from(block); copy.fill(32, 148, 156);
+        if (recorded !== copy.reduce((sum, byte) => sum + byte, 0)) throw new Error("Invalid archive checksum");
+        const size = Number.parseInt(block.toString("ascii", 124, 136).replaceAll("\0", "").trim(), 8);
+        if (!Number.isSafeInteger(size) || size < 0 || offset + 512 + size > tar.length) throw new Error("Invalid archive size");
+        files.set(name, tar.subarray(offset + 512, offset + 512 + size));
+        offset += 512 + Math.ceil(size / 512) * 512;
+    }
+    if (!files.has("workspace.json")) throw new Error("Archive has no workspace.json");
+    let workspace;
+    try { workspace = JSON.parse(files.get("workspace.json").toString("utf8")); }
+    catch { throw new Error("Archive has invalid workspace.json"); }
+    return {workspace, images: files};
+}