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/lib/SharesBar.svelte

@@ -77,7 +77,7 @@
     border: 1px solid var(--border);
     border-radius: 10px;
     background: var(--bg-card);
-    padding: 0.85rem 1.25rem;
+    padding: 1.1rem 1.5rem;
   }
   .content {
     position: relative;
@@ -85,29 +85,29 @@
     display: flex;
     align-items: center;
     justify-content: center;
-    gap: 1.5rem;
+    gap: 2rem;
   }
   .title {
     margin: 0;
-    font-size: 0.9rem;
-    font-weight: 600;
+    font-size: 1.1rem;
+    font-weight: 700;
     text-transform: uppercase;
-    letter-spacing: 0.04em;
+    letter-spacing: 0.05em;
     color: var(--fg);
   }
   .stats {
     display: flex;
     align-items: center;
-    gap: 1.5rem;
+    gap: 2rem;
     flex-wrap: wrap;
   }
   .group {
     display: flex;
     align-items: baseline;
-    gap: 0.4em;
+    gap: 0.5em;
   }
   .group-label {
-    font-size: 0.75rem;
+    font-size: 0.85rem;
     text-transform: uppercase;
     letter-spacing: 0.04em;
     color: var(--fg-dim);
@@ -115,31 +115,31 @@
   }
   .accepted {
     color: var(--good);
-    font-weight: 600;
-    font-size: 1.1rem;
+    font-weight: 700;
+    font-size: 1.4rem;
     font-variant-numeric: tabular-nums;
   }
   .rejected {
     color: var(--bad);
-    font-weight: 600;
-    font-size: 1.1rem;
+    font-weight: 700;
+    font-size: 1.4rem;
     font-variant-numeric: tabular-nums;
   }
   .sep {
     color: var(--fg-dim);
-    font-size: 0.9rem;
+    font-size: 1.1rem;
   }
   .pct {
-    font-size: 1.1rem;
+    font-size: 1.05rem;
     color: var(--fg-dim);
-    margin-left: 0.3em;
+    margin-left: 0.4em;
   }
   .pct.warn {
     color: var(--bad);
   }
   .divider {
     width: 1px;
-    height: 1.6rem;
+    height: 2rem;
     background: var(--border);
   }