refactor: rename the "provider" term in VFS config to "service"

[sxyazi]

Follow-up to #3396

This PR corrects terminology in the virtual filesystem before the official release by introducing a new "service" concept to distinguish it from the existing "provider":

• Provider: the backend implementation of the VFS, e.g. SFTP
• Service: a concrete instance of a provider, i.e. a specific provider setting, for SFTP this is the set of parameters like host, port, user, etc.

If you used the providers field in vfs.toml, please replace it with services. For example:

- [providers.my-server]
+ [services.my-server]
type = "sftp"
host = "1.2.3.4"
user = "root"
port = 22

The latest nightly documentation has been updated to reflect this change: https://yazi-rs.github.io/docs/next/configuration/vfs

[Copilot]

Pull request overview

This PR performs a comprehensive refactoring of the VFS system, introducing clearer terminology by distinguishing between "provider" (the backend implementation) and "service" (a concrete instance with specific settings). Beyond the terminology change, it also refactors symlink operations and path handling utilities.

• Renamed Provider enum to Service and ProviderSftp struct to ServiceSftp throughout the codebase
• Changed the VFS configuration field from providers to services in vfs.toml
• Refactored symlink function signatures to use a more intuitive parameter order (link first, then original/target)
• Converted URL-based relative path calculation to path-based calculation for improved clarity

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
yazi-config/src/vfs/mod.rs	Updated module declaration to import service instead of provider
yazi-config/src/vfs/service.rs	Renamed Provider enum to Service and ProviderSftp to ServiceSftp
yazi-config/src/vfs/vfs.rs	Renamed providers field to services and updated method names and error messages
yazi-vfs/src/provider/sftp/conn.rs	Updated type references from ProviderSftp to ServiceSftp
yazi-vfs/src/provider/sftp/gate.rs	Updated type and method references to use ServiceSftp and Vfs::service()
yazi-vfs/src/provider/sftp/mod.rs	Updated static hashmap type to use ServiceSftp
yazi-vfs/src/provider/sftp/sftp.rs	Updated struct field and method call to use ServiceSftp and Vfs::service()
yazi-vfs/src/provider/provider.rs	Reordered symlink function parameters to place link before original for better API ergonomics
yazi-scheduler/src/file/file.rs	Updated symlink call to match new parameter order and refactored relative path handling to use path-based calculation
yazi-shared/src/path/cow.rs	Added PartialEq implementation for PathCow to enable equality comparisons
yazi-fs/src/path/relative.rs	Refactored from URL-based to path-based relative calculation with comprehensive test coverage
yazi-fs/src/path/path.rs	Removed obsolete URL-based relative path tests
yazi-plugin/preset/plugins/mime-remote.lua	Added check to skip dummy files in MIME detection
yazi-plugin/preset/plugins/fzf.lua	Added virtual filesystem detection to prevent fzf usage in unsupported contexts

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