Fix RSS feeds failing to load due to strict request header validation.

[Sizexian]

Description

This PR fixes an issue where some RSS feeds fail to load because of strict server-side header checks.
Previously, fetch requests from the renderer process were blocked by CORS or strict header validation on some sites.

Changes

1. Moved RSS, favicon, and article fetches from the renderer to the main process.
2. Exposed safe bridges for fetch operations using ipcMain/ipcRenderer.
3. Added proper request headers (User-Agent, Accept, Accept-Language) to increase compatibility with stricter servers.
4. Fixed charset handling and decoding logic via decodeFetchResponse to handle non-UTF8 feeds.
5. Updated related fetch functions (RSS, favicon, article HTML) to return correct types (string, boolean, ArrayBuffer).

Notes

• This does NOT bypass WAFs or paywalls like IEEE Xplore. Sites with strict bot detection may still block requests.
• The fix ensures that feeds and favicons are fetched correctly in the majority of cases, improving stability and user experience.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested

• Verified that RSS feeds with strict header validation now load successfully.
• Verified that favicons are fetched and validated correctly.
• Verified that full article HTML is returned as ArrayBuffer and correctly decoded.
• Manual testing on multiple sites including UTF-8 and non-UTF-8 RSS feeds.

Screenshots (if applicable)

Linked issues / References

• Inspired by a post on 52pojie forum: Fluent Reader RSS Reader 418 Error Fix