fix(web): rollback optimistic feed state on failed transaction

[Olajcodes]

Summary

When the feed applies an optimistic like/follow/tip mutation, the UI updates immediately for an instant feel. However, if the underlying on-chain transaction reports failed, the optimistic state was never reverted — the feed kept showing a "liked" heart, a "Following" label, or a bumped tip total that was never persisted, and a later unrelated refetch would snap the UI back to server truth, causing a visible flash/unsync.

This PR adds snapshot/rollback support to optimisticStore.ts and wires Feed.tsx to clear its transient "pending" styling when a rollback happens, so the UI always reconciles back to the pre-mutation state on failure.

Type of Change

• Bug fix
• New feature
• Contract change (logic, storage, or API)
• Documentation update
• Refactor / chore

Changes

apps/web/src/lib/optimisticStore.ts

• Added snapshot capture for like, follow, and tip state before each optimistic mutation:
  • snapshotLikeState(key), snapshotFollowState(key), snapshotTipState(key)
• Added the rollback path that restores the pre-mutation snapshot for the exact post/user affected, clears the snapshot, notifies subscribed React components (so they re-render with the restored state), and emits a rolled-back event:
  • rollbackLikeState, rollbackFollowState, rollbackTipState
• Added a RolledBackEvent type { kind: "follow" | "like" | "tip"; key: string } and an onRolledBack(listener) subscription (returns an unsubscribe function) so components can clear transient "pending" styling.
• Snapshots are deep clones (JSON-serialization based) so they never share references with live state.
• Added clearLikeState / clearTipState counterparts for symmetry and test cleanup.

apps/web/src/components/Feed.tsx

• Tracks per-post "pending" state (pendingLikes) while an optimistic write is in flight, showing a transient Liking... label and pulse styling.
• Subscribes to OptimisticStore.onRolledBack and clears the pending styling for the affected post when a like/follow write is rolled back after a failed transaction.

apps/web/src/lib/__tests__/optimisticStore.test.ts (new)

• Success path — committed optimistic state is retained (no rollback).
• Failure path — a failed like/follow/tip restores the pre-mutation snapshot and emits a rolled-back event.
• No-prior-state — when there was no optimistic entry before the write, the rollback drops the entry so the UI falls back to server truth.
• Partial rollback ordering — rolling back one post/user does not disturb unrelated snapshots, and sequenced rollbacks restore their own state independently.
• Deep-clone independence — mutating live state after snapshotting cannot corrupt the stored snapshot.
• onRolledBack unsubscribe stops delivering events.

Testing Done

• New tests added for changed behaviour: 9 unit tests in apps/web/src/lib/__tests__/optimisticStore.test.ts — all passing.
• tsc --noEmit from apps/web — the only reported errors are pre-existing ones in packages/sdk (generated client: scvU64/scvSymbol/bigint typing), which are out of scope of this issue; no errors in the files changed here.
• cargo test passes — N/A (frontend-only change)
• Manually verified on Testnet (if applicable) — not possible in this environment
• TypeScript SDK tests passed (if applicable) — N/A

Checklist

• Changes are focused — one concern per PR (optimistic rollback)
• If a contract function was added or changed, the README API table is updated — N/A (frontend only)
• No unresolved merge conflicts
• No secrets or private keys committed

Related Issue

Closes #1191

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