feat: lemmas for String.front?

[TwoFX]

This PR proves theorem front?_eq {s : String} : s.front? = s.toList.head? and related results.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 83c6f6e5acddc2a29d71740814f75dfeb9ad2333 --onto e6df474dd9c3ad0e21771eaa808c53f66222216d. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-24 15:15:02)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 83c6f6e5acddc2a29d71740814f75dfeb9ad2333 --onto cfa8c5a036d6990635c6ec50b02d0e806995cec3. You can force reference manual CI using the force-manual-ci label. (2026-03-24 15:15:04)