Import instead of link rel

Replace Google Fonts with @import inside for better Gmail compatibility
Problem

Currently, Frappe Letters injects Google Fonts using:

<link rel="stylesheet" href="https://fonts.googleapis.com/css2?...">

This works in browsers, but many email clients ignore or remove <link> elements. In particular, Gmail strips these tags from the email HTML, causing the custom font to fall back to the default font.

Proposed solution

Instead of generating a <link> tag, generate:

<style>
@import url('https://fonts.googleapis.com/css2?...');

Gmail preserves blocks and accepts @import rules, allowing the font to be loaded correctly.

Current output
<link rel="stylesheet" href="https://fonts.googleapis.com/css2?...">
Proposed output
<style>
@import url('https://fonts.googleapis.com/css2?...');

Why this change?
Improves compatibility with Gmail.
Keeps the implementation simple.
Does not change the public API.
Requires only a small change in fonts.py.

Start in fonts.py and locate the code that generates the Google Fonts link element. Change the generated output to a style block containing the equivalent @import rule, without changing the public API. Verify that the resulting email HTML matches the proposed output and preserves the Google Fonts URL.

Written by the indexing model from the issue text.