vt_data auto-update, persist setting, add stdlib/fmt fallbacks & CI fixes (7 commits)

[No0ne558]

Short summary

• This PR bundles the last 7 commits which together:
  • Ensure vt_data auto-update respects the authoritative fixed settings and UI toggle (including when restarting from non-server displays).
  • Persist the UI toggle immediately so restarts honor the user preference.
  • Prevent creation of preset/default employees when employee.dat already exists.
  • Add robust fallbacks for std::to_underlying and formatting APIs so the project builds across multiple stdlib/toolchain combinations.
  • Misc CI/build fixes (includes <algorithm> for std::clamp, guard macros, and {fmt}/snprintf fallback).

Commits included

• 4147b5a ci: fallback to {fmt} or snprintf when std::format unavailable (2026-04-09)
  • Files: cpp23_utils.hh
• 0970e08 ci: include so std::clamp is available (2026-04-09)
  • Files: cpp23_utils.hh
• bec572e ci: guard std::to_underlying with feature-test macro (2026-04-09)
  • Files: vt_enum_utils.hh
• 6e76d97 fix(enum): fallback for std::to_underlying on older stdlibs (2026-04-09)
  • Files: vt_enum_utils.hh
• 52526cc chore: commit remaining changes (2026-04-09)
  • Files: manager.cc
• 474afbf docs(changelog): add default-employee creation note (2026-04-09)
  • Files: changelog.md
• 2096430 chore(changelog): document vt_data auto-update fixes; respect fixed settings and non-server restarts (2026-04-09)
  • Files: changelog.md, manager.cc, settings_zone.cc

What changed (high-level)

• Startup/update flow:
  • Startup autoupdate now requires both the early .viewtouch_config.autoupdate flag and the authoritative fixed settings auto_update_vt_data to run.
  • FindVTData() now respects auto_update_vt_data and will skip downloads when disabled.
  • Added logging for autoupdate suppression and restart origin for auditing.
• Settings/UI:
  • The auto-update UI switch now calls settings->Save() immediately so restarts see the updated preference.
• Default users:
  • Preset/default employees are created only when employee.dat is missing.
• Build/compatibility:
  • Added a portable fallback for converting enums to underlying values when std::to_underlying isn't available.
  • Guarded uses of std::to_underlying with the feature-test macro.
  • Added formatting utility fallbacks: use std::format when available, fallback to {fmt} if present, else degrade to an snprintf-based fallback so older toolchains can compile.
  • Included <algorithm> to ensure std::clamp is available on older toolchains.

Files touched (aggregate)

• manager.cc
• settings_zone.cc
• vt_enum_utils.hh
• cpp23_utils.hh
• changelog.md