feat: RPC caching, background decrypt import, fast-lane peers, mining fix

RPC client:
- Add call() overload with per-call timeout parameter
- z_exportwallet uses 300s, z_importwallet uses 1200s timeout

Decrypt wallet (app_security.cpp, app.cpp):
- Show per-step and overall elapsed timers during decrypt flow
- Reduce dialog to 5 steps; close before key import begins
- Run z_importwallet on detached background thread
- Add pulsing "Importing keys..." status bar indicator
- Report success/failure via notifications instead of dialog

RPC caching (app_network.cpp, app.h):
- Cache z_viewtransaction results in viewtx_cache_ across refresh cycles
- Skip RPC calls for already-cached txids (biggest perf win)
- Build confirmed_tx_cache_ for deeply-confirmed transactions
- Clear all caches on disconnect
- Remove unused refreshTransactions() dead code

Peers (app_network.cpp, peers_tab.cpp):
- Route refreshPeerInfo() through fast_worker_ to avoid head-of-line blocking
- Replace footer "Refresh Peers" button with ICON_MD_REFRESH in toggle header
- Refresh button triggers both peer list and full blockchain data refresh

Mining (mining_tab.cpp):
- Allow pool mining toggle when blockchain is not synced
- Pool mining only needs xmrig, not local daemon sync

src/rpc/rpc_client.h

@@ -61,6 +61,15 @@ public:
      */
     json call(const std::string& method, const json& params = json::array());
 
+    /**
+     * @brief Make a raw RPC call with a custom timeout
+     * @param method RPC method name
+     * @param params Method parameters
+     * @param timeoutSec Timeout in seconds (0 = no timeout)
+     * @return JSON response or throws on error
+     */
+    json call(const std::string& method, const json& params, long timeoutSec);
+
     /**
      * @brief Make a raw RPC call and return the result field as a string
      * @param method RPC method name