Comprehensive MCP Apps docs, string CallTool resolution (#3575)

* Comprehensive MCP Apps docs, string CallTool resolution, bump prefab-ui >=0.13.0

Rewrites the apps documentation as a learning journey: overview → Prefab apps
→ FastMCPApp → patterns → dev tools → custom HTML. Adds a new FastMCPApp page
covering composable apps with @app.tool()/@app.ui(), CallTool, forms, actions,
and composition. Teaches Rx() and set_initial_state() as the primary state API.

Adds string-based CallTool resolution so CallTool("save_contact") resolves to
the tool's global key, matching callable ref behavior. Requires prefab-ui 0.13.0
which passes strings through the tool resolver.

* Detect ambiguous string CallTool resolution across apps

* Simplify string name registry to plain dict (last-write-wins)

docs/docs.json

@@ -193,6 +193,7 @@
                 "pages": [
                   "apps/overview",
                   "apps/prefab",
+                  "apps/interactive-apps",
                   "apps/patterns",
                   "apps/development",
                   "apps/low-level"