generate the dashboards at build time instead of committing them

The JSON is machine-written and the generator beside it is the only source of
truth, so committing both only created a way for them to drift -- which is what
the dashboards-current check existed to police. modules/dashboards.nix now runs
gen_zfs.py / gen_smart.py in the derivation, so a stale dashboard cannot be
built at all and the check is gone.

Note there is no third option: flakes copy only git-tracked files into the
store, so a gitignored dashboards/*.json cannot be read from disk either. It is
generate, or commit.

Verified equivalent -- both generated dashboards are byte-identical to the JSON
this commit deletes.

Adds a dashboards-nonempty check. Factoring the shared jq invocation out first
introduced a bug it caught: the multi-line string ended in a newline, so the
callers' `< $src > "$out"` landed on a line of its own as a separate command,
truncating every dashboard to zero bytes while the build still succeeded. All
four provisioned dashboards were empty. jqFixup is now a single line, and the
check asserts each dashboard is non-empty and has a title, uid and panels.

Both failure modes are exercised: an empty dashboard and one with no panels each
fail the check. It also asserts it found four files, because the first version
globbed *.json against linkFarm entries named without the suffix and passed
vacuously.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JKczN1f66kmLxNBLX37voQ

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