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

ui/src/stores/selection.svelte.ts

@@ -3,6 +3,7 @@
 //   #/                    -> dashboard
 //   #/user/<address>      -> per-user page
 //   #/worker/<workername> -> per-worker page
+//   #/accelerator         -> transaction accelerator
 //
 // selectUser / selectWorker / clearSelection just mutate the hash;
 // a hashchange listener reads it back into reactive state so any
@@ -10,29 +11,34 @@
 
 const USER_PREFIX = "#/user/";
 const WORKER_PREFIX = "#/worker/";
+const ACCELERATOR_HASH = "#/accelerator";
 
-type Selection = { user: string | null; worker: string | null };
+type Selection = { user: string | null; worker: string | null; page: string | null };
 
 export const selection = $state<Selection>(readHash());
 
 function readHash(): Selection {
-  if (typeof window === "undefined") return { user: null, worker: null };
+  if (typeof window === "undefined") return { user: null, worker: null, page: null };
   const h = window.location.hash;
+  if (h === ACCELERATOR_HASH) {
+    return { user: null, worker: null, page: "accelerator" };
+  }
   if (h.startsWith(WORKER_PREFIX)) {
     const w = decodeURIComponent(h.slice(WORKER_PREFIX.length));
-    return { user: null, worker: w || null };
+    return { user: null, worker: w || null, page: null };
   }
   if (h.startsWith(USER_PREFIX)) {
     const u = decodeURIComponent(h.slice(USER_PREFIX.length));
-    return { user: u || null, worker: null };
+    return { user: u || null, worker: null, page: null };
   }
-  return { user: null, worker: null };
+  return { user: null, worker: null, page: null };
 }
 
 function syncFromHash(): void {
   const parsed = readHash();
   selection.user = parsed.user;
   selection.worker = parsed.worker;
+  selection.page = parsed.page;
 }
 
 if (typeof window !== "undefined") {
@@ -47,11 +53,16 @@ export function selectWorker(workername: string): void {
   window.location.hash = WORKER_PREFIX + encodeURIComponent(workername);
 }
 
+export function selectAccelerator(): void {
+  window.location.hash = ACCELERATOR_HASH;
+}
+
 export function clearSelection(): void {
   if (
     window.history.length > 1 &&
     (window.location.hash.startsWith(USER_PREFIX) ||
-      window.location.hash.startsWith(WORKER_PREFIX))
+      window.location.hash.startsWith(WORKER_PREFIX) ||
+      window.location.hash === ACCELERATOR_HASH)
   ) {
     window.history.back();
   } else {