Add Safari support and public beta rollout tooling

[iamwey]

Relevant issue: Safari support / public beta groundwork
Tested in browser: Safari (local validation); Chrome/Firefox smoke still pending on this branch

Summary

• add a Safari build target, Safari manifest, and Xcode conversion/validation flow
• add Safari capability fallbacks for unsupported history/download/auth paths
• add Safari runtime diagnostics and debug tooling so failures are reportable
• add Safari public-beta rollout docs, issue intake, and macOS CI validation
• add cross-browser regression guardrails for the shared runtime wrappers touched by the Safari port

Diff classification

• Safari-only work: safari/manifest.json, build/safariConvert.js, Safari docs, macOS Safari CI validation
• Shared wrappers: apiToPromise, foreground/background messaging transport, capabilities, toolbar action handling, diagnostics helpers
• Shared behavior behind capability gates: history/download/auth wrappers, cloud backup provider capability exposure, showImages and filteReddit Safari fallbacks

Automated verification

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

Manual browser smoke before review-ready

• Chrome on old Reddit: injects, settings save/reload, toolbar style toggle works on a styled subreddit, showImages works, downloads remain managed, history-dependent behavior still works
• Firefox on old Reddit: injects, settings save/reload, toolbar style toggle works on a styled subreddit, showImages works, downloads remain managed, history-dependent behavior still works, Firefox-specific auth/storage behavior still matches expectations
• Safari on old Reddit: smoke checklist in CONTRIBUTING.md completed on the current branch/TestFlight candidate

Safari limitations currently documented

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

[iamwey]

Superseded by #5587, which contains the trimmed upstream-only Safari port. Closing this mixed rollout/distribution PR so review can continue on the clean runtime-essential branch.