feat(ui): generic Skeleton component and skeleton loading states

[RitvikSardana]

Description

Adds a single generic <Skeleton /> component and uses it to replace spinners / blank screens with content-shaped loading states across the agent portal.

The component (desk/src/components/Skeleton/) has no per-screen configuration and two modes:

• Bone mode — no slot: renders one gray pulse block, shaped via variant (line / circle / box) and width/height or utility classes. This is the primitive everything else composes.
• Mask mode — slot + loading: renders the slotted component inert (inert, aria-hidden), walks the DOM and turns every leaf node (text, images, icons, buttons, inputs) into a gray pulse block at its real rendered size. When loading turns false the slot renders untouched. Meant for presentational components fed mock props.

The folder mirrors frappe-ui component conventions (Skeleton.vue, types.ts, index.ts), so it can be lifted upstream into frappe-ui as-is later.

Screens wired up (all bone composition, since these screens fetch their own data):

Screen	Before	After
All list views (Tickets, Contacts, Customers, Call Logs, KB — via ListViewBuilder)	centered spinner on first load	table skeleton: header + 12 rows with avatar and varied-width lines
Agent ticket page (TicketAgent.vue)	small spinner	full-layout skeleton: header bar, 3 conversation bubbles, sidebar field pairs
Customer / Contact detail pages	blank screen while doc loads	shared DetailPageSkeleton: avatar header, 4 stat cards, tabs, list lines

Skeletons show only on first load; reloads keep existing rows visible as before. Line widths vary deterministically ((row + column) % 4) so rows read organic without randomness.

How to test

1. cd desk && yarn dev, open the agent portal.
2. Throttle the network in devtools (Slow 3G helps make the states visible).
3. Visit Tickets / Contacts / Customers / Knowledge Base — first load shows the table skeleton, then rows swap in place.
4. Open a single ticket — layout skeleton with conversation bubbles and sidebar until the doc loads.
5. Open a customer and a contact record — previously a blank screen, now the detail-page skeleton (square avatar for customer, circle for contact).
6. Reload a list that already has data — no skeleton flash; existing rows stay visible while refreshing.

yarn build verified after each commit.

Related issue

No linked issue — raised from an internal improvement idea; happy to open one if needed.