refactor(ui): delete dead button code (buttons.h + ApplyTactile)

UI-standardization audit found two dead, contradictory "how to draw a button"
surfaces with zero call sites:
- src/ui/material/components/buttons.h (353 lines: TextButton/Outlined/Contained/
  Button(spec)/IconButton/FAB) — included by 3 mining files but never used.
- ApplyTactile (draw_helpers.h) — a retrofit-tactile-onto-plain-button wrapper,
  never called.
Remove the file, the 3 dead includes, and ApplyTactile. TactileButton/StyledButton
remain the canonical button helpers. No behavior change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ui/windows/mining_tab.cpp

@@ -22,7 +22,6 @@
 #include "../material/type.h"
 #include "../material/draw_helpers.h"
 #include "../material/colors.h"
-#include "../material/components/buttons.h"
 #include "../effects/low_spec.h"
 #include "../layout.h"
 #include "../notifications.h"