security(csp): version-safe inline-script nonce wrapper + strengthen PO/MO instructions

[TheWitness]

Summary

Part of the fleet-wide CSP-nonce rollout (mirrors the pilot Cacti/plugin_mactrack#374).

1. Version-safe CSP nonce on inline JavaScript. Adds a wrapper
   plugin_<name>_csp_nonce() and applies it to every inline <script> block, so
   pages stay compatible with Cacti's Content-Security-Policy nonce enforcement.
2. Strengthened i18n instruction in .github/copilot-instructions.md — the
   .po/.mo files are never committed (Weblate owns them); only cacti.pot.

Wrapper (cross-version safe)

function plugin_<name>_csp_nonce(): string {
	if (class_exists('CactiSecureHeaders')) {
		return CactiSecureHeaders::getNonceAttribute();
	}

	return '';
}

Emits the per-request nonce="..." on Cacti releases that ship CactiSecureHeaders
(present on both develop and 1.2.x), and '' on older releases — so the tag
stays valid either way. The wrapper lives in the plugin's functions library where one
exists, otherwise in setup.php (loaded on every Cacti page by the plugin loader).

Also

• tests/Unit/CspNonceTest.php covers the empty-string fallback, the string
  return type, and delegation to CactiSecureHeaders.
• Where the plugin emits its own external <script src> / CSS <link>, those go
  through Cacti's get_md5_include_js() / get_md5_include_css() helpers (automatic
  nonce + md5 cache-buster).
• locales/po/cacti.pot regenerated for shifted source line references; the
  per-language .po/.mo catalogs are intentionally not committed.

[Copilot]

Copilot review overview

🟡 Changes recommended

Isolate the fallback test from global bootstrap state so it remains valid when CactiSecureHeaders is loaded.

Review effort: Lite
Findings: 1

Open (1)

• Avoid global bootstrap state in class-absent fallback test · New

What changed in this PR

Adds version-safe CSP nonce support for FlowView inline scripts and strengthens PO/MO handling guidance.

Changes:

• Adds and applies a CSP nonce wrapper.
• Uses Cacti include helpers for CSS and JavaScript assets.
• Adds nonce tests and updates translation references and documentation.

File	Description
tests/​Unit/​CspNonceTest.php	Adds CSP nonce tests; fallback coverage should be isolated from bootstrap-loaded classes.
setup.php	Adds the nonce wrapper and include helper usage.
locales/​po/​cacti.pot	Updates generated source references.
functions.php	Adds nonces to inline scripts.
flowview_schedules.php	Adds nonces to page scripts.
flowview_filters.php	Adds nonces to page scripts.
flowview_devices.php	Adds nonces to page scripts.
flowview_databases.php	Adds nonces to page scripts.
CHANGELOG.md	Documents the security change.
.github/​copilot-instructions.md	Clarifies PO/MO catalog ownership.

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