feat(nodes): add per-node TLS verification mode for self-signed certs (#4757)

Adds a per-node TLS verification mode to the Add/Edit Node dialog so the panel can reach nodes that serve HTTPS with a self-signed certificate:

- verify (default): normal CA validation.
- skip: InsecureSkipVerify, with a clear UI warning that it drops MITM protection.
- pin: validates the leaf certificate's SHA-256 (base64 or hex) via VerifyConnection while bypassing the default chain/name check — keeps MITM protection for self-signed certs, the secure alternative to skip.

New Node model fields tlsVerifyMode + pinnedCertSha256 (gorm auto-migrated). Probe() selects the HTTP client per node via nodeHTTPClientFor, keeping the SSRF-guarded dialer. A new POST /panel/api/nodes/certFingerprint endpoint (FetchCertFingerprint) lets the UI fetch and pin the node's current certificate in one click. Endpoint documented in api-docs/openapi; i18n added across all locales. Verified end-to-end in Docker (verify rejects, skip bypasses, fetch matches, pin accepts correct / rejects wrong).

web/translation/en-US.json

@@ -869,7 +869,19 @@
         "updateStarted": "Panel update started",
         "updateResult": "Update triggered on {ok} node(s), {failed} failed",
         "updateNoneEligible": "Select at least one online, enabled node"
-      }
+      },
+      "tlsVerifyMode": "TLS verification",
+      "tlsVerifyModeHint": "How the panel validates the node's HTTPS certificate. Pin or Skip are for self-signed certs (https nodes only).",
+      "tlsVerify": "Verify (default CA)",
+      "tlsPin": "Pin certificate (SHA-256)",
+      "tlsSkip": "Skip verification",
+      "tlsSkipWarning": "Skipping verification removes protection against man-in-the-middle attacks — the API token could be intercepted. Prefer pinning the certificate.",
+      "pinnedCert": "Pinned certificate SHA-256",
+      "pinnedCertHint": "Base64 or hex SHA-256 of the node's certificate. Use Fetch to read it from the node now.",
+      "pinnedCertPlaceholder": "base64 or hex SHA-256",
+      "fetchPin": "Fetch",
+      "pinFetched": "Fetched the node's current certificate",
+      "pinFetchFailed": "Could not fetch the certificate"
     },
     "settings": {
       "title": "Panel Settings",