refactor(mining): extract the Earnings card into mining_earnings.{h,cpp} (audit #10, slice 1)

First incremental slice of decomposing the 2628-line mining_tab.cpp monolith (one giant
RenderMiningTabContent function). The ~636-line "Earnings" card section is moved verbatim into
RenderMiningEarnings(); mining_tab.cpp is now 1992 lines and calls it with the immediate-mode
layout context as parameters (draw list, fonts, scale/spacing, glass spec, pool-mode flag).

Behavior-preserving by construction: the body is byte-identical (the only additions are a
`const bool s_pool_mode = poolMode` alias and a local scratch `buf` so the moved code keeps its
original identifiers). The earnings-filter static moved with the card it belongs to. The
compiler surfaced every enclosing dependency, which became explicit parameters.

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

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

CMakeLists.txt

@@ -442,6 +442,7 @@ set(APP_SOURCES
     src/ui/windows/receive_tab.cpp
     src/ui/windows/transactions_tab.cpp
     src/ui/windows/mining_tab.cpp
+    src/ui/windows/mining_earnings.cpp
     src/ui/windows/mining_benchmark.cpp
     src/ui/windows/mining_pool_panel.cpp
     src/ui/windows/mining_tab_helpers.cpp