feat: merge legacy play dashboard features into prediction terminal

[Spagero763]

feat: merge legacy /play dashboard features into the prediction terminal

Closes #130

What this does

High-value panels previously living only on /play (LegacyDashboard) are now
part of the primary /dashboard terminal, and /play is deprecated and
redirected. This gives one primary route with chart, round lifecycle, and social
features, and removes the maintainer confusion between the two dashboards.

Changes

• src/pages/Dashboard.tsx
  • Round lifecycle: ported RoundTimeline (round state timeline) into the
    terminal. The price chart and end-round modal were already present.
  • Connection status: ported the ConnectionStatus banner (socket + SSE
    round-update health) so users see when live updates are degraded.
  • Social: added an opt-in Community chat toggle that mounts
    ChatSidebar. It is hidden by default to keep the fintech terminal
    uncluttered; ChatSidebar is a self-positioned fixed slide-over, so toggling
    it causes no layout shift to the terminal grid.
• src/App.tsx — /play now permanently redirects to /dashboard
  (<Navigate replace />); the unused LegacyDashboard lazy import was removed.
  The LegacyDashboard component file is retained for reference only.
• README.md — documents the consolidation and the /play deprecation under
  Project structure notes, linking issue Merge legacy play dashboard features into prediction terminal #130 for the remaining social/notification
  consolidation.

Acceptance criteria

• Chart and round lifecycle integrated into /dashboard (price chart +
  RoundTimeline + connection status + end-round modal).
• Chat/notifications available (opt-in toggle) and the remaining
  consolidation is intentionally scoped with an issue link in the README.
• /play deprecated/redirected with a note in the README.
• Layout remains responsive (grid unchanged; chat is a self-contained
  fixed slide-over with its own mobile behavior).

Verification

• tsc -b: no new type errors from the changed files.
• eslint: no new errors/warnings from the changed files.
• vitest: this branch produces no new test failures. The suite has pre-existing
  failures on main (the educationApi mock gap in Dashboard.terminal.test.tsx
  and the CountdownTimer/RoundCard/useRoundCountdown timer tests) — verified
  identical on a clean main checkout, so they are unrelated to this PR.

Note: tsc -b also reports one pre-existing error unrelated to this PR
(src/components/CountdownTimer.tsx: unused React import) that already fails
on main. Left untouched to keep this PR scoped to #130.

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