Refactor duplicated code into shared helpers

[devin-ai-integration]

Summary

calc.py repeated the same tk.Button(...) + .grid(...) construction ~11 times and the same tk.Label(...) + .pack(...) 3 times, each with identical styling (fg=LABEL_COLOR, borderwidth=0, padx=24, etc.). This collapses those into two shared helpers and removes several smaller copy-paste blocks. Pure refactor — no behavior change (verified with a headless smoke test covering arithmetic, x², √x, and memory M+/MR).

Net: -81 / +50 lines.

Key changes:

• New helpers on Calculator:
  • _create_button(text, command, row, column, bg=OFF_WHITE, font=DEFAULT_FONT_STYLE, columnspan=1) — builds + grids a styled button and returns it.
  • _create_label(text, font) — builds + packs a styled display label.
• All create_*_button methods now delegate to _create_button. create_operator_buttons uses enumerate instead of a manual counter.
• The four near-identical memory buttons (create_mc/mr/mplus/mminus_button, which even used \ line continuations) are replaced by a data-driven create_memory_buttons() iterating a new self.memory_buttons = {"MC": (1,0), ...} map.
• square/sqrt/perc shared the self.current_expression = str(eval(...)); self.update_label() idiom — extracted into _apply(expression):

  def _apply(self, expression):
      self.current_expression = str(eval(expression))
      self.update_label()

• Repeated self.memol.config(text="M: "+str(self.mem)) (in MS/M+/M-) extracted into _update_memory_label().
• add_to_expression had three branches that all did the same append; simplified to one guarded append (only skip a . when one is already present):

  if self.entrou == 0:
      if value != '.' or '.' not in self.current_expression:
          self.current_expression += str(value)

Note: the pre-existing iconphoto(... calc3.png) reference (file not in repo) is left untouched as out of scope.

Link to Devin session: https://app.devin.ai/sessions/1fcf820df6684c45bdbd4f6851761fc1
Requested by: @martlyma

[devin-ai-integration]

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