feat(client): add transport.websocketPath option for custom websocket handshake path

[6rock]

WHY

Closes #5388

When transport.protocol is websocket or wss, frpc always performs the websocket handshake on a hard-coded path /~!frp (pkg/util/net/websocket.go), with no way to configure it. This makes it hard to place frps behind a reverse proxy (e.g. nginx) that routes traffic by path — for example sharing port 443 with other services, or running multiple frps instances behind a single entrypoint.

This PR adds a client-side transport.websocketPath option:

• Defaults to /~!frp when empty — fully backward compatible, existing configs are unaffected.
• When set (must start with /), the client uses it for the websocket/wss handshake.
• The server needs no changes: the reverse proxy is responsible for forwarding the custom path back to frps' default /~!frp.

Example:

transport.protocol = "wss"
transport.websocketPath = "/frp"

nginx then matches location /frp, rewrites it to /~!frp, and proxies to frps.

Changes

• Add WebsocketPath to the v1 ClientTransportConfig and the legacy client config (with legacy→v1 conversion).
• Thread the path through DialHookWebsocket, defaulting to /~!frp when empty (consistent with the existing empty-parameter defaulting in that function).
• Validate that the path starts with / (both v1 and legacy validation).
• Update frpc_full_example.toml, the legacy ini example, and Release.md.

The change is client-side only; go build / go vet and pkg/config/... tests pass.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (9 files)

• Release.md
• client/connector.go
• conf/frpc_full_example.toml
• conf/legacy/frpc_legacy_full.ini
• pkg/config/legacy/client.go
• pkg/config/legacy/conversion.go
• pkg/config/v1/client.go
• pkg/config/v1/validation/client.go
• pkg/util/net/dial.go

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_{Reviewed by gpt-5.5-20260423 · Input: 73K · Output: 5.1K · Cached: 338.2K}