[REQUEST] Support POSIX timezone formats (e.g., BRT3) in Web UI settings and documentation

Problem Statement

When using minimal container images (such as browserless/chrome, which runs on a lightweight Ubuntu 24.04 base without tzdata), standard IANA timezone identifiers like America/Sao_Paulo fail to resolve inside the container. Since /usr/share/zoneinfo/America/Sao_Paulo is missing, glibc/musl falls back to UTC time.

Manually adding a POSIX timezone format such as "timeZone": "BRT3" directly into state/settings.json works flawlessly under the hood. Because POSIX formats calculate the offset mathematically directly in the C standard library (glibc/musl), it requires zero files, no tzdata packages, and no /etc/timezone volume mounts.

However, the Runtipi Web UI currently restricts or does not facilitate entering POSIX timezone strings.

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Proposed Solution

1. Web UI Support:

   • Allow entering or selecting POSIX-compliant timezone formats (e.g., BRT3, EST5EDT, UTC0, <-03>3) in the Runtipi Web UI settings instead of strictly validating against the IANA timezone database.
   • If a dropdown is used, allow a custom write-in option or include standard POSIX representations.

2. Documentation:

   • Document POSIX timezone format support in Runtipi's configuration docs as a recommended universal alternative for users running minimal/slim containers that lack the tzdata package.

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Why this is useful

• Universal Compatibility: POSIX timezone strings work out-of-the-box on virtually all Linux images (Ubuntu, Debian, Alpine, BusyBox) without needing any installed packages or host volume mounts.
• Zero Breaking Changes: Under the hood, Runtipi already passes the value from settings.json into the containers via TZ, meaning this only requires frontend validation/support in the Web UI.

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Environment

• Runtipi Version: v4.10.1
• Host OS: DietPi v10.7.2