fix(#548): derive volunteer match status from live opportunities

[nadavosa]

Summary

• VolunteerHeader was reading volunteer.statusMatch — a stored DB field that the BE never updates when opportunity-volunteer statuses change
• Now derives match status directly from the ["volunteer-opportunities", volunteerId] React Query cache, which VolunteerOpportunities already keeps up-to-date
• No extra network request: React Query deduplicates the identical query key

Derivation rules (highest priority wins):

Opportunity status	Displayed match status
Any MATCHED or ACTIVE	Matched
Any PENDING	Pending match
Only PAST	Past
None	No matches

Closes #548

Test plan

• Volunteer with no opportunities → header shows "No matches"
• Volunteer with a pending opportunity → header shows "Pending match"
• Volunteer with a matched opportunity → header shows "Matched"
• Mark a pending opportunity as Matched → header updates immediately
• Mark active back to past → header updates to "Past" if no other active/matched opps
• Volunteer profile still loads and engagement status row still works

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