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/ui/windows/mining_tab.cpp

@@ -608,7 +608,11 @@ void RenderMiningTab(App* app)
             bool isSyncing  = state.sync.syncing;
             bool poolBlockedBySolo = s_pool_mode && mining.generate && !state.pool_mining.xmrig_running;
             bool isToggling = app->isMiningToggleInProgress();
-            bool disabled   = !app->isConnected() || isToggling || isSyncing || poolBlockedBySolo;
+            // Pool mining connects to an external pool via xmrig — it does not
+            // need the local blockchain synced or even the daemon connected.
+            bool disabled = s_pool_mode
+                ? (isToggling || poolBlockedBySolo)
+                : (!app->isConnected() || isToggling || isSyncing);
 
             // Glass panel background with state-dependent tint
             GlassPanelSpec btnGlass;
@@ -733,7 +737,7 @@ void RenderMiningTab(App* app)
                     ImGui::SetMouseCursor(ImGuiMouseCursor_Hand);
                 if (isToggling)
                     ImGui::SetTooltip(isMiningActive ? "Stopping miner..." : "Starting miner...");
-                else if (isSyncing)
+                else if (isSyncing && !s_pool_mode)
                     ImGui::SetTooltip("Syncing blockchain... (%.1f%%)", state.sync.verification_progress * 100.0);
                 else if (poolBlockedBySolo)
                     ImGui::SetTooltip("Stop solo mining before starting pool mining");