Add transaction accelerator (prioritisetransaction UI + API)

Full-stack feature for boosting transactions via bitcoind's
prioritisetransaction RPC:

Backend:
- New accelerator package with Accelerate, Cancel, MaxFeerate, List,
  and background Cleanup goroutine (removes confirmed/dropped txs)
- RPC wrappers: GetMempoolEntry, PrioritiseTransaction,
  GetRawMempoolVerbose, IsRPCError helpers
- SQLite boosted_txs table for persistence across restarts
- Revenue impact measured via getblocktemplate before/after comparison
- Hard cap at 2000 sat/vB; MaxFeerate uses fees.base (not modified)
  to ignore our own prior priority adjustments

Frontend:
- Rocket icon in header with 10s jiggle animation
- AcceleratorPage with flame-gradient border, txid input, feerate
  input, "Prioritize above all" button with spinner, boost list with
  cancel buttons, and dismissible error/success messages
- Hash-based routing (#/accelerator)
- SharesBar font size bump

api/internal/store/blocks.go

@@ -109,6 +109,10 @@ func Open(path string) (*BlockStore, error) {
 		_ = db.Close()
 		return nil, err
 	}
+	if err := s.migrateAccelerator(); err != nil {
+		_ = db.Close()
+		return nil, fmt.Errorf("store: accelerator schema: %w", err)
+	}
 	return s, nil
 }