Add Safari support

[iamwey]

Supersedes #5586.

Summary

• add a Safari build target, manifest, and Xcode conversion/validation flow
• add Safari runtime and capability fallbacks for unsupported APIs
• fix Safari settings opening, media expandos, v.redd.it, and styled comment-page layout
• add shared-wrapper and Safari runtime tests for the touched compatibility paths

Not in scope

• public beta or TestFlight rollout
• diagnostics page or export flow
• fork branding or support routing
• CI, docs, changelog, or contributor-process changes

Automated verification

• yarn test
• yarn eslint
• yarn build --browsers=all
• yarn safari:validate

Manual smoke before review-ready

• Chrome on old Reddit: injects, settings persist, style toggle works, showImages works, history-dependent behavior still works
• Firefox on old Reddit: same smoke, plus Firefox-specific auth/storage behavior still matches expectations
• Safari on old Reddit: injects, settings open from Reddit pages, showImages works, v.redd.it works, styled comment pages behave correctly

Known Safari limitations

• no extension-managed history integration
• downloads fall back to opening the asset in a new tab
• automatic cloud backups remain disabled unless Safari auth behavior is confirmed

Reviewer notes

This branch was intentionally trimmed to runtime-essential Safari support only so the review does not include rollout tooling or fork-specific support surfaces.