fix: migrate NGA image hosts

[BugenZhao]

What changed

• Add a centralized resource URL resolver that maps legacy NGA image hosts to img.nga.cn or img4.nga.cn based on the resource path.
• Route post media, attachments, avatars, and forum icons through the resolver while keeping backend and internal-link preferences unchanged.
• Update the forum icon service constant and network-backed fixtures to the canonical image hosts.
• Preserve third-party absolute image URLs.

Root cause

NGA stopped resolving img.nga.178.com, while MNGA still used it as the fixed attachment base. The alternate backend candidates do not expose equivalent working img.<backend> endpoints, so image resources require canonical asset hosts independent of the selected API backend.

User impact

Restores NGA-hosted images, attachment previews, avatars, media links, and forum icons. Historical absolute URLs using retired NGA image domains are migrated at load time.

Validation

• make swiftformat
• cargo test -p service (9 passed, 46 ignored external-state tests)
• cargo clippy
• make build (iPhone 17 Pro, iOS 26.2 Simulator; Build Succeeded)
• Direct live probes for canonical attachment and forum icon paths returned HTTP 200

Fixes #399

[Copilot]

Pull request overview

This PR restores broken NGA-hosted images by introducing a centralized URL resolver on the Swift side and updating hardcoded/test fixture image hostnames (Rust + Swift) to the canonical img.nga.cn / img4.nga.cn domains, while preserving non-NGA third-party absolute URLs.

Changes:

• Added URLs.resourceURL / URLs.attachmentURL to rewrite legacy NGA image domains to canonical hosts (and force HTTPS), based on resource path.
• Routed avatars, forum icons, and attachment/media URL construction through the resolver in Swift UI/content rendering paths.
• Updated Rust forum icon base constant and updated network-backed fixture URLs to use canonical image hosts.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
logic/text/src/content.rs	Updates a parsing test fixture image URL to the canonical host.
logic/service/src/post.rs	Updates a network-backed test probe image URL to the canonical host.
logic/service/src/constants.rs	Switches forum icon base URL to https://img4.nga.cn/....
app/Shared/Views/UserView.swift	Routes avatar loading through URLs.resourceURL.
app/Shared/Views/UserMenuView.swift	Routes menu avatar loading through URLs.resourceURL and keys view identity off the resolved URL.
app/Shared/Views/TopicListView.swift	Updates preview forum icon URL to canonical host.
app/Shared/Views/PostContentView.swift	Updates preview image URL to canonical host.
app/Shared/Views/ForumIconView.swift	Routes forum icon loading through URLs.resourceURL and keys identity off the resolved URL.
app/Shared/Views/AttachmentsView.swift	Uses URLs.attachmentURL for attachment navigation/viewing.
app/Shared/Utilities/URLs.swift	Adds centralized legacy-host migration logic and updates attachment base host.
app/Shared/Utilities/ContentCombiner.swift	Uses URLs.attachmentURL when constructing media/attachment URLs from parsed content.
app/Shared/Protos/Extensions.swift	Updates user avatar fixture URL to canonical host.
app/Shared/Models/AttachmentsModel.swift	Resolves attachment URLs via URLs.attachmentURL and matches against preview URLs by equality.

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