fix: stop AuthContext.getSession clobbering session-derived user (#223)

[vedant7007]

Summary

The `getSession().then(...)` handler in `AuthContext.tsx` called `setUser` twice back-to-back. React batches both calls (same microtask) so the second wins, unconditionally discarding whatever the first computed. The first call already covered all three cases (session → `toAppUser`, guest-mode → `guestUser`, else → `null`) so the second was pure noise.

Effects:

• authenticated users saw a logged-out flash on every load until `onAuthStateChange` fired again to restore session
• `ProtectedRoute`, `GameProvider`, and Community fetch could race against a `null` user on the first render, causing spurious redirects to `/login` and empty states
• in guest mode, the merge-prompt path was skipped because `login` was never actually called with the session's user

What changed

Deleted the redundant `setUser(isGuestMode() ? guestUser : null)` on line 158. That's it — 1 removed line + a small comment explaining the invariant.

Test plan

• verified the preceding ternary covers all three states
• no other file references the discarded value
• once maintainer approves: manually sign in, close tab, reopen, confirm dashboard renders with the user's profile immediately with no logged-out flash

Fixes #223

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[vedant7007]

hey! heads up — the failing Verify Codebase check is red because of a pre-existing issue on master: src/App.tsx:13 imports ./pages/Auth but no Auth file exists in src/pages/. every PR opened right now fails on the same import resolution error (see [Failed to resolve import "./pages/Auth" from "src/App.tsx"] in the run log).

this PR doesn't touch App.tsx or anything in src/pages/, so nothing here is causing it. happy to open a separate PR restoring the Auth page (or wiring the route to whatever the intended replacement is) if you can point me at what should live there. thanks!