refactor(mining): extract the Controls/CPU-grid card into mining_controls.{h,cpp} (audit #10, slice 3)

Third and largest slice of decomposing mining_tab.cpp. The ~843-line "Controls" card (CPU-core
grid + drag-to-select, mining start/stop button, benchmark + miner-update controls) is moved
verbatim into RenderMiningControls(). mining_tab.cpp is now 839 lines (was 2628 originally).

The most coupled section, so mutated state is passed BY REFERENCE — the benchmark
(ThreadBenchmark&), selected thread count (int&), and drag state (bool&/int&) — with local
reference aliases so the body stays byte-identical and interactions (drag, benchmark, start/stop)
behave exactly as before. Read-only context is passed by value/const; the compiler verified
const-correctness. Local statics inside the block moved with it.

Verified: full-node + Windows + lite build, tests, hygiene, no startup crash.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CMakeLists.txt

@@ -444,6 +444,7 @@ set(APP_SOURCES
     src/ui/windows/mining_tab.cpp
     src/ui/windows/mining_earnings.cpp
     src/ui/windows/mining_stats.cpp
+    src/ui/windows/mining_controls.cpp
     src/ui/windows/mining_benchmark.cpp
     src/ui/windows/mining_pool_panel.cpp
     src/ui/windows/mining_tab_helpers.cpp