feat: add copy button to code blocks

[k1LoW]

Summary

• Add a copy-to-clipboard button on code blocks, shown on hover (top-right corner)
• Uses GitHub Octicons (copy-16 / check-16) for consistent icon style
• Button background matches the dark code block theme (github-dark)

[github-actions]

Code Metrics Report

	main (748c698)	#32 (2504990)	+/-
Coverage	14.1%	14.1%	0.0%
Code to Test Ratio	1:0.1	1:0.1	-0.1
Test Execution Time	22s	22s	0s

Details

  |                     | main (748c698) | #32 (2504990) | +/-  |
  |---------------------|----------------|---------------|------|
  | Coverage            |          14.1% |         14.1% | 0.0% |
  |   Files             |              9 |             9 |    0 |
  |   Lines             |            593 |           593 |    0 |
  |   Covered           |             84 |            84 |    0 |
- | Code to Test Ratio  |          1:0.1 |         1:0.1 | -0.1 |
  |   Code              |           1548 |          1588 |  +40 |
  |   Test              |            284 |           284 |    0 |
  | Test Execution Time |            22s |           22s |   0s |

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Reported by octocov