feat(web): make footer legal links configurable

Add a "main.footer-links" config option so the footer Imprint and
Privacy Policy links can point at internal pages (default) or external
URLs. External http(s) targets open in a new tab; empty/unset values
fall back to the built-in /imprint and /privacy routes, keeping the
existing ./var/*.tmpl override mechanism intact.

Closes #517

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Entire-Checkpoint: c5dca72c848f

configs/config.json

@@ -18,6 +18,10 @@
       "trigger": 180,
       "resolutions": [240, 60]
     },
+    "footer-links": {
+      "imprint": "/imprint",
+      "privacy": "/privacy"
+    },
     "api-subjects": {
       "subject-job-event": "cc.job.event",
       "subject-node-state": "cc.node.state"