Add configurable web transport paths

[durck]

Summary

Adds configurable web transport paths for WebSocket and HTTP(S) polling transports. Defaults remain /ws and /push.

Details

• Adds server --ws-path and --push-path options.
• Adds client --ws-path and --push-path options.
• Lets link --ws-path and link --push-path bake paths into generated clients.
• Lets the Docker entrypoint pass RSSH_WS_PATH and RSSH_PUSH_PATH into server startup.
• Normalizes configured paths with a leading slash and no trailing slash.
• Updates mux HTTP classification and WebSocket unwrap to use configured paths.

Validation

• go test ./pkg/transport ./internal/client ./pkg/mux
• go test -vet=off ./cmd/client ./cmd/server ./internal/server ./internal/server/commands ./internal/server/webserver
• bash -n docker-entrypoint.sh via Git Bash
• git diff --check

Server package validation uses -vet=off because upstream currently has unrelated non-constant format string vet findings; this PR intentionally does not include formatting-only cleanup.