Add configurable script loading method to keep tracking out of critical rendering path

Problem/Motivation

The Umami tracking script is currently loaded with async defer attributes directly in the HTML. While this is non-blocking, the browser still discovers and fetches the script during parsing, which means it appears in the critical request chain and can negatively impact page speed scores (e.g. Lighthouse, PageSpeed Insights).

Proposed resolution

Add a configurable "Script loading method" setting with two options:

• Deferred (default) — Injects the tracking script dynamically via window.addEventListener('load', ...), so the script is only fetched after all page resources have finished loading. This keeps analytics completely out of the critical rendering path.
• Async — Preserves the current async defer behavior for sites that prefer it.

Remaining tasks

• New script_mode config key (default: onload)
• Config schema and install config updated
• Settings form: radio buttons with per-option descriptions
• hook_page_attachments: conditional logic based on the selected mode