[springmin]

Email links previously called webbrowser::open directly, which cannot launch a browser on hosts without desktop helper processes.

PlatformContext now carries a cloneable ExternalUrlOpener. Native hosts install their URL bridge on the context; the application uses it for email links and browser-based OAuth, including reconnection, calendar authorization, and retries. Host errors propagate without falling back to desktop launchers.

Email links retain their scheme allowlist and in-app mailto handling. OAuth validates HTTPS addresses before dispatch. Default desktop behavior is preserved, including the Linux portal and AppImage library-path handling for authorization.

Hosts migrating from the initial version of this PR replace set_external_url_opener(bridge::open_url) with:

platform.external_urls = flectar_mail::ExternalUrlOpener::new(bridge::open_url);

The callback must accept calls from the application event loop and OAuth worker threads. The contract and migration are documented in platform/README.md. The stacked OpenHarmony host in #85 needs this assignment after constructing its mutable context.

The dependency follow-up updates ammonia to patched version 4.1.8 in both lockfiles and the Flatpak source manifest, and documents three maintenance-notice exceptions for crates merged into zbus 6. Slint and desktop portal dependencies still require zbus 5; vulnerability findings and all other warnings remain denied.

Validation

• Formatting and platform-boundary lint pass locally.
• Workspace Clippy with all targets and warnings denied passes locally.
• Regression coverage includes URL policy, error propagation, independent host instances, worker-thread dispatch, reader callbacks, in-app compose, and OAuth retries.
• The URL-opener commit passed the full local suite (948 passed, zero failed, eight ignored), the build without default features, and all six build/lint/test CI jobs.
• Both locked dependency graphs pass the RustSec audit after the dependency follow-up. Android lockfile metadata validation passes.
• With patched ammonia, all core targets pass (611 passed, zero failed, two ignored), and core Clippy passes with warnings denied.
• All 13 release tests and Blitz/Flatpak source verification pass locally.
• Cross-platform CI is rerunning on the final commit.

Contribution and licensing

• I have the right to submit this work.
• Third-party dependencies/assets: none added.
• Disclosed generative-tool assistance: the change was developed with an
  AI coding agent (LLM); I reviewed every line, ran the checks above, and
  tested the behavior as described.
• No credentials, private mailbox data, signing material, or unnecessary
  personal information included.

[iamdarkle]

Thanks for this contribution, the platform opener is a useful addition. I've moved it into PlatformContext, shared it with browser-based OAuth, and added regression coverage. CI is green; once I resolve the conflict with main in quality.yml and CI passes again, this is ready to merge.

For #85, please replace the global set_external_url_opener registration with platform.external_urls = ExternalUrlOpener::new(bridge::open_url); on the PlatformContext returned by app_private, and verify link opening and OAuth on a device