web: full hostContext delivery + live host-context-changed updates

[cliffhall]

Summary

Deliver the full hostContext to MCP App views and keep it live:

• Initial hostContext carries theme, styles (Mantine design tokens), displayMode, availableDisplayModes, and containerDimensions (today: only theme).
• Live changes — theme flip, container resize, maximize — push to the running view via the SDK's sendHostContextChange(patch) (no hand-maintained snapshot ref).

Why

hostContext is how a widget adapts to its host; carrying only theme means widgets tested against the inspector never see the fields real hosts send. Using the SDK's patch mechanism keeps us aligned with @modelcontextprotocol/ext-apps.

Reference implementation (PR #1510)

Re-implement informed by these changes at 33fac3f:

• clients/web/src/components/elements/AppRenderer/AppRenderer.tsx — hostContext assembly on init, theme/style observation, sendHostContextChange pushes
• clients/web/src/components/elements/AppRenderer/createAppBridgeFactory.ts — hostContext plumbing hunks
• clients/web/src/components/elements/AppRenderer/AppRenderer.test.tsx — renderer tests (+432)

Depends on

• hostContext utilities (AppRenderer/hostContext.ts)
• CSP enforcement + resource-error surfacing in the bridge factory (same files; land first)

Wave 2 lane — sequential with the other AppRenderer/AppsScreen issues.

Notes

• Manual check: theme toggle in the inspector should deliver host-context-changed to a running mcp_app_demo widget.
• Coverage gate ≥90 on all four dimensions.

Part of the PR #1510 decomposition (see tracking issue).