test(demo): add full guided-demo walkthrough and targeted demo tests

Adds one comprehensive Playwright test that walks a fresh Operations
Manager session through all 8 Demo Guide steps performing the real
action at each one, then restores the environment. Writing it surfaced
a real desktop layout bug: the Demo Guide's fixed side panel overlapped
main content with no reflow, making the return form's "Review return"
button unclickable while the guide was open at ordinary viewport widths.
Fixed by reserving layout space via a guide-open class. Also adds mobile
bottom-sheet, keyboard-reachability, and console-error checks.

frontend/src/styles.css

@@ -327,6 +327,11 @@ details summary { cursor: pointer; color: var(--teal-dark); }.data-table details
 .demo-guide-progress-pill { padding: 1px 6px; color: var(--teal-dark); background: white; border-radius: 999px; font-size: .6rem; }
 
 .demo-guide-panel { position: fixed; z-index: 40; top: 0; right: 0; width: min(400px, 92vw); height: 100vh; display: flex; flex-direction: column; gap: 14px; padding: 20px; background: var(--surface); border-left: 1px solid var(--line); box-shadow: var(--shadow-float); overflow-y: auto; }
+@media (min-width: 701px) {
+  /* The guide panel is a fixed right-side overlay at this width -- push page content
+     aside while it's open so it never sits underneath and blocks actionable buttons. */
+  .app-workspace.guide-open { padding-right: min(400px, 92vw); }
+}
 .demo-guide-header { display: flex; align-items: flex-start; justify-content: space-between; gap: 10px; }
 .demo-guide-kicker { margin: 0 0 4px; color: var(--teal-dark); font-size: .64rem; font-weight: 700; text-transform: uppercase; letter-spacing: .08em; }
 .demo-guide-header h2 { margin: 0; font-size: 1.05rem; letter-spacing: -.015em; }