feat: warn when a newer PiHoleShell release is available

[mikemadeja]

Summary

• Adds a best-effort, cached check on module import that compares the installed version against GitHub's latest release (GET /repos/mikemadeja/PiHoleShell/releases/latest) and prints a warning if a newer one exists
• Designed to never slow down or break Import-Module:
  • Skipped entirely for a dev/source checkout - importing the bare .psm1 (as every test in this repo's suite does) gets PowerShell's auto-assigned version 0.0, and importing via the manifest gets the 0.0.0 placeholder that's only replaced at publish time; both are explicitly treated as "not a real install" and skipped, so this never fires during CI/local test runs
  • Cached to disk for 24 hours so repeat imports don't hit the network or GitHub's unauthenticated rate limit
  • Opt-out via $env:PIHOLESHELL_SKIP_UPDATE_CHECK for automation that wants no network calls at all
  • GitHub's "latest release" endpoint already excludes prereleases/drafts, so the dev-latest build published on every push to develop is never mistaken for a real update
  • Any failure (network, parsing) is swallowed silently via try/catch - it can never break a real import
• Documented in the README's Installation section

Test plan

• Unit tests (8, mocked, no server dependency): dev-checkout versions (both 0.0 and 0.0.0) skip the check, opt-out env var skips the check, warns and caches on a real newer version, no warning when already current/newer, cache reuse within the interval, re-check after cache expiry, never throws on API failure
• Manually confirmed against the live GitHub API that an old version warns correctly and a current/newer version stays silent
• Confirmed importing the module from source (both Import-Module .\PiHoleShell.psm1 and via a bare .psm1) produces zero warnings and zero network calls
• PSScriptAnalyzer clean
• Full non-integration unit suite passes (12/12)

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