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/bitcoind/rpc.go

@@ -276,3 +276,73 @@ func (c *RPC) GetBlockTemplate(ctx context.Context) (*BlockTemplate, error) {
 	}
 	return &out, nil
 }
+
+// ---- mempool / priority methods ----
+
+// MempoolEntryFees mirrors the "fees" object inside getmempoolentry.
+type MempoolEntryFees struct {
+	Base       float64 `json:"base"`
+	Modified   float64 `json:"modified"`
+	Ancestor   float64 `json:"ancestor"`
+	Descendant float64 `json:"descendant"`
+}
+
+// MempoolEntry is the result of getmempoolentry <txid>.
+type MempoolEntry struct {
+	Vsize int64            `json:"vsize"`
+	Fees  MempoolEntryFees `json:"fees"`
+}
+
+// GetMempoolEntry returns the mempool entry for a transaction.
+// Returns rpcError code -5 if the tx is not in the mempool.
+func (c *RPC) GetMempoolEntry(ctx context.Context, txid string) (*MempoolEntry, error) {
+	var out MempoolEntry
+	if err := c.Call(ctx, "getmempoolentry", []any{txid}, &out); err != nil {
+		return nil, err
+	}
+	return &out, nil
+}
+
+// PrioritiseTransaction adjusts a transaction's apparent fee for block
+// template selection. feeDelta is in satoshis (positive = boost,
+// negative = de-prioritise).
+func (c *RPC) PrioritiseTransaction(ctx context.Context, txid string, feeDelta int64) error {
+	var ok bool
+	if err := c.Call(ctx, "prioritisetransaction", []any{txid, 0, feeDelta}, &ok); err != nil {
+		return err
+	}
+	return nil
+}
+
+// RawMempoolEntry is the verbose form of a mempool entry from getrawmempool true.
+type RawMempoolEntry struct {
+	Vsize int64            `json:"vsize"`
+	Fees  MempoolEntryFees `json:"fees"`
+}
+
+// GetRawMempoolVerbose returns all mempool transactions with their details.
+func (c *RPC) GetRawMempoolVerbose(ctx context.Context) (map[string]RawMempoolEntry, error) {
+	var out map[string]RawMempoolEntry
+	if err := c.Call(ctx, "getrawmempool", []any{true}, &out); err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+// IsRPCError checks if err is an rpcError with a specific code.
+func IsRPCError(err error, code int) bool {
+	var rerr *rpcError
+	if errors.As(err, &rerr) {
+		return rerr.Code == code
+	}
+	return false
+}
+
+// RPCErrorMessage extracts the message from an rpcError, or the error string.
+func RPCErrorMessage(err error) string {
+	var rerr *rpcError
+	if errors.As(err, &rerr) {
+		return rerr.Message
+	}
+	return err.Error()
+}