refactor(session): scope pending plan approval reads

[beruro]

Summary

• add a session-scoped usePendingPlanApproval hook backed by a derived atom family
• migrate pending-plan read consumers while retaining pendingPlanApprovalsAtom as the single write/source-of-truth path
• add focused coverage for session isolation and empty-session behavior

Supersedes closed PR #318. This rebuild starts from current develop.

Validation

• 24 focused Vitest tests passed
• tsc --noEmit --pretty false
• ESLint on all 10 changed files
• git diff --check
• GitHub Frontend and Rust CI passed

Scope notes

• preserves the existing rehydrate race protection on develop
• does not duplicate or replace usePlanApproval
• does not add a second writable approval atom