web: honor ui/download-file with confirmation and http(s) allowlist

[cliffhall]

Summary

Honor ui/download-file requests from MCP App views: with user confirmation, either write an embedded resource (text/blob) to disk or open an http(s)-only resource_link in a new tab. Labels shown in the confirmation are sanitized.

Why

ui/download-file is part of the ext-apps host surface and is currently silently dropped, so widgets using it appear broken in the inspector.

Reference implementation (PR #1510)

Re-implement informed by these changes at 33fac3f:

• clients/web/src/components/elements/AppRenderer/createAppBridgeFactory.ts — ondownloadfile handler, base64ToBytes(), sanitizeDownloadLabel(), describeDownloadItem(), downloadResourceItem()
• clients/web/src/lib/downloadFile.ts — consumed library (own issue)
• clients/web/src/components/elements/AppRenderer/createAppBridgeFactory.test.ts — download-path tests

Depends on

• downloadFile.ts enhancements (library)
• CSP enforcement + resource-error surfacing in the bridge factory (same file; land first)

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

Notes

• Security posture: confirmation required; resource_link restricted to http(s) (no javascript:/data:/file:); sanitize any widget-supplied text shown in host UI.
• Coverage gate ≥90 on all four dimensions.

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