Validate image tag format in config and improve error message

[anisaoshafi]

Motivation

We don't validate the version tag in config.toml so users discover later in the flow some tags are not possible.
Also the error message was not pointing to the real issue.

Changes

• Reject tags that don't conform to Docker's tag format rules at config load (invalid chars, leading ./-, >128 chars). Fails fast before any network or Docker operation.
• Normalize tags with zero-padded calendar months (2026.04 → suggest 2026.4) before license validation
• The suggested valid tag in error messages is computed from the previous calendar month, so it's a reasonable recent version.
• License API error improvement: when the license API returns 400 with a licensing.license.format body, the error now says "unsupported image tag" instead of the generic token/subscription message, which is more actionable for the user.

a) Before // inaccurate message

b) Now: tag="3.99.99" // valid tag regex, but does't exist: license server complains

c) Now: tag="2026.04" // valid tag, previously failed with error a)

[gtsiolis]

Cross-posting relevant decision to redirect this to support zero padding.

[anisaoshafi]

Cross-posting relevant decision to redirect this to support zero padding.

Adjusted according to discussion [1] 🙏🏼

[gtsiolis]

UX LGTM, works as expected. 💯