docs(apps): specify the v2 Apps tab (#1265)

Adds the missing Apps tab to the v2 UX specification. Apps are the subset
of tools whose `_meta.ui.resourceUri` advertises a UI resource (per
`@modelcontextprotocol/ext-apps`). The screen mirrors the legacy v1 layout:
a sidebar of apps and a right pane that toggles between an input form
(before launch) and the embedded MCP App via `AppBridge`.

- v2_ux.md: add `[Apps]` to the navigation tab list and a new "Apps Screen"
  subsection with both states (input form / running app).
- v2_ux_features.md: full Apps Screen feature spec — ASCII diagrams,
  detection rule, presentational contract for `AppsScreen` / `AppRenderer`
  (imperative ref shape so the screen stays "dumb" and the wiring layer
  drives the bridge).
- v2_screenshots.md: link the two reference PNGs from the legacy v1 client.
- v2_ux_storybook.md: add `AppListItem` (2.3a), `AppDetailPanel` (2.5a),
  `AppRenderer` (2.5b), and `AppsScreen` (3.2a); update Build Order.

Implementation work tracked on the v2 board: #1260–#1264.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

specification/v2_screenshots.md

@@ -51,6 +51,18 @@ Tool list with annotations (user, read-only, destructive, long-running), paramet
 
 ![Tools Screen](screenshots/shadcn-tools.png)
 
+### Apps Screen
+
+The subset of tools that are MCP Apps — tools with `_meta.ui.resourceUri`. Two-panel layout: list of apps on the left, input form (before launch) or embedded app iframe (after launch) on the right.
+
+**Input form state** (from legacy v1 reference):
+
+![Apps Screen — Input Form](screenshots/apps-tab-form.png)
+
+**Running app state** (from legacy v1 reference):
+
+![Apps Screen — Running App](screenshots/apps-tab-app.png)
+
 ### Resources Screen
 Accordion layout with Resources, Templates, and Subscriptions sections.