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/material/draw_helpers.h

@@ -260,18 +260,6 @@ inline void DrawButtonGlassOverlay(ImDrawList* dl, const ImVec2& bMin,
     DrawTactileOverlay(dl, bMin, bMax, rounding, active);
 }
 
-// Convenience: call right after any ImGui::Button() / SmallButton() call
-// to add tactile depth.  Uses the last item rect automatically.
-inline void ApplyTactile(ImDrawList* dl = nullptr)
-{
-    if (!dl) dl = ImGui::GetWindowDrawList();
-    ImVec2 bMin = ImGui::GetItemRectMin();
-    ImVec2 bMax = ImGui::GetItemRectMax();
-    float rounding = ImGui::GetStyle().FrameRounding;
-    bool active = ImGui::IsItemActive();
-    DrawTactileOverlay(dl, bMin, bMax, rounding, active);
-}
-
 // ── Button font tier helper ─────────────────────────────────────────────
 // Resolves an int tier (0=sm, 1=md/default, 2=lg) to the matching ImFont*.
 // Passing -1 or any out-of-range value returns the default button font.