Replace crib position input with interactive slider modal

[samdaw13]

Summary

• Replace the plain number TextInput for crib position with a new CribPositionModal that lets users visually slide the crib along the ciphertext using arrow buttons
• Modal highlights conflicting positions in red (where a letter would encrypt to itself, an Enigma impossibility) and disables the Confirm button when invalid
• Centralize page padding in the Page template component — removes per-page paddingVertical overrides so all screens get consistent padding: 16 from a single source
• Add findConflictIndices helper to crib-analysis.tsx for detecting letter-to-self conflicts at a given position

Test plan

• npm run lint passes (0 errors)
• npm test passes (149 tests)
• Open Break Cipher, enter ciphertext + crib, tap position button, verify modal opens
• Verify arrows navigate positions, conflicts show red, confirm is disabled on conflict
• Verify confirm stores position (1-indexed on button label), clear resets to undefined
• Verify all screens still have correct padding after Page refactor

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