Refactor: remove redundant useEffects and derived state across core views

https://react.dev/learn/you-might-not-need-an-effect

Refactor Header, AccountBar, PostPage, Subplebbit, and SubmitPage to derive values during render instead of syncing them with effects. Remove effect-driven state, keeping effects only for real side‑effects; add memoization where it meaningfully reduces work. Maintain current behavior while reducing re-renders and simplifying components.