[TD-03b] Separate hospital/technician signing via partially-signed transaction hand-off

[licette32]

Implement true dual-wallet signing for complete_maintenance so hospital and technician
resolve to two distinct pubkeys, replacing the current state where both roles use the
same connected wallet. Follow-up to TD-03 (#3) — the localStorage keypair was removed
there, but role separation itself remains unresolved.

• Add a builder that constructs the unsigned completeMaintenance instruction(s),
  sets hospital as fee payer, and returns a partially-signed Transaction once the
  hospital wallet signs.
• Add serialize/deserialize helpers so the partially-signed transaction can be
  transferred out of the hospital's session and loaded into the technician's session.
• Add a hand-off UI step in the hospital flow: after triggering "complete maintenance,"
  display the serialized transaction with a copy action instead of sending it directly.
• Add a corresponding load step in the technician flow: paste input, decoded summary
  for review (asset, hospital, escrow amount) before signing, then submit.
• Verify the account metas in the parsed transaction match PDAs re-derived from the
  transfer payload before allowing the technician to sign (no blind signing).
• Handle transaction expiration (recentBlockhash validity window) with a visible
  countdown on both sides.

Acceptance Criteria:

• hospital and technician resolve to two distinct pubkeys in a completed
  complete_maintenance transaction when tested with two separate wallets.
• Technician sees a decoded, verified summary before signing — never a raw blob.
• No new dependencies.