[codex] Add Signal launch hardening

[MaxGhenis]

Summary

• Adds launch-quality Signal support surfaces: live bridge wiring, pairing/status APIs, Signal desktop import/backfill paths, media/reaction handling, and tests.
• Hardens the inbox UI around multi-route contacts, platform chips/icons, long-thread rendering/windowing, bottom pin behavior, stale avatar hydration, notifications, reactions, read receipts, and platform management.
• Refreshes the website screenshots/copy for the current design and adds release diagnostics plus a release checklist.

Validation

• git diff --check
• go test ./...
• swift build from macos/OpenMessage
• npm run build from site
• npm run test:e2e (43/43 passed)

Notes

• Website screenshots were visually checked for private chat leakage; they use synthetic demo content.
• .tmp-openmessage-recover-filtered.sql remains local-only via .git/info/exclude and was not staged.
• A subagent review was attempted but did not return before timeout; local release-gate checks passed.