Expand E2E coverage beyond smoke test

## Summary
Expand end-to-end coverage so RFChat has durable browser-level confidence in the highest-risk user flows for a single-guild deployment.

## Scope
- Cover authentication, invite/onboarding, guild navigation, message send/edit/delete, and key permission-denied states.
- Cover settings workflows that materially change user-visible behavior, especially moderation, notifications, channel management, and role/permission administration.
- Cover media, reactions, threads, and reconnect-sensitive flows once those features exist.
- Keep the suite web-desktop-first, deterministic, and runnable in CI.

## Acceptance Criteria
- [ ] Browser tests cover the core happy path from sign-in to active chatting.
- [ ] Browser tests cover at least one forbidden/permission-denied path for each major privileged workflow.
- [ ] Browser tests cover reconnect- or refresh-sensitive flows where correctness can regress without unit tests catching it.
- [ ] The suite is isolated, repeatable, and fast enough to run in CI on every change that touches critical user journeys.

## Operational Notes
- Tests should use seeded data and stable selectors, not brittle visual text matching.
- Self-hosted deployments need the suite to validate production-like behavior, not only local dev shortcuts.

Start by locating the existing browser smoke test and the CI entry point, then map the authentication, onboarding, guild, messaging, permissions, settings, media, reactions, threads, and reconnect flows described here. Done means deterministic seeded-data coverage for the core happy path and representative denied or reconnect-sensitive paths, runnable repeatedly in CI with stable selectors.

Written by the indexing model from the issue text.