feat: non-blocking warmup — connect during daemon initialization

Instead of blocking the entire UI with "Activating best chain..." until
the daemon finishes warmup, treat warmup responses as a successful
connection. The wallet now:

- Sets connected=true + warming_up=true when daemon returns RPC -28
- Shows warmup status with block progress in the loading overlay
- Polls getinfo every few seconds to detect warmup completion
- Allows Console, Peers, Settings tabs during warmup
- Shows orange status indicator with warmup message in status bar
- Skips balance/tx/address refresh until warmup completes
- Triggers full data refresh once daemon is ready

Also: fix curl handle/header leak on reconnect, fill in empty
externalDetected error branch, bump version to v1.2.0 in build scripts.

src/data/wallet_state.h

@@ -180,6 +180,8 @@ struct PoolMiningState {
 struct WalletState {
     // Connection
     bool connected = false;
+    bool warming_up = false;         // daemon reachable but in RPC warmup (error -28)
+    std::string warmup_status;       // e.g. "Activating best chain..."
     int daemon_version = 0;
     std::string daemon_subversion;
     int protocol_version = 0;
@@ -250,6 +252,8 @@ struct WalletState {
     
     void clear() {
         connected = false;
+        warming_up = false;
+        warmup_status.clear();
         daemon_version = 0;
         daemon_subversion.clear();
         protocol_version = 0;