feat: Add snackbar extension point

[cheneyveron]

Description

Add a new web runtime extension point: app.runtime.snackbars.

This allows plugins to render custom runtime UI in the existing snackbar area. It was triggered by an archiver plugin I'm working on: opencloud-file-archiver, which uses this extension point to display archive job status and runtime actions such as cancel.

This PR is intentionally scoped to the snackbar extension point only. Other extension enhancements are split into separate PRs.

How Has This Been Tested?

• test environment:

  • Debian 13 x86_64
  • OpenCloud image: opencloudeu/opencloud:7.2.0-posix-fastpath-20260704, bundled reva
  • Related extension tested with: opencloud-file-archiver@d2d565a

• Type Check & Build

pnpm check:types
Result: passed.

pnpm build
Result: passed.

• Manual test:
  • Registered snack bar by my extension is showing correct information at the right place and time.
  • The buttons to clear events and expand/collapse are working.
  • Existing features, such as upload, download, preview, and client sync are working.

Types of changes

• Enhancement (a change that doesn't break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)