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/App.svelte

@@ -11,6 +11,7 @@
   import BestShares from "./lib/BestShares.svelte";
   import UserDetailPage from "./lib/UserDetailPage.svelte";
   import WorkerDetailPage from "./lib/WorkerDetailPage.svelte";
+  import AcceleratorPage from "./lib/AcceleratorPage.svelte";
   import SharesBar from "./lib/SharesBar.svelte";
   import BlockFoundAnimation from "./lib/BlockFoundAnimation.svelte";
 
@@ -34,6 +35,8 @@
         <div class="stat-sub bad">{snap.error}</div>
       {/if}
     </div>
+  {:else if selection.page === "accelerator"}
+    <AcceleratorPage />
   {:else if selection.worker}
     <WorkerDetailPage />
   {:else if selection.user}