refactor(mining): extract the Hashrate+Stats card into mining_stats.{h,cpp} (audit #10, slice 2)

Second slice of decomposing mining_tab.cpp. The ~313-line "Hashrate + Stats" card (stat values +
hashrate chart / live-log view) is moved verbatim into RenderMiningStats(); mining_tab.cpp is now
1680 lines (was 1992 after slice 1, 2628 originally). Body byte-identical apart from a s_pool_mode
alias; the chart/log toggle statics (s_show_pool_log/s_show_solo_log) moved with the card, and the
log buffer was already a function-local static. No App dependency in this section.

Verified: full-node + Windows + lite build, tests, hygiene, clean smoke start. Pending hands-on
visual check before the next slice.

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

CMakeLists.txt

@@ -443,6 +443,7 @@ set(APP_SOURCES
     src/ui/windows/transactions_tab.cpp
     src/ui/windows/mining_tab.cpp
     src/ui/windows/mining_earnings.cpp
+    src/ui/windows/mining_stats.cpp
     src/ui/windows/mining_benchmark.cpp
     src/ui/windows/mining_pool_panel.cpp
     src/ui/windows/mining_tab_helpers.cpp