server/plugin: don't panic on whitespace in Jitsi Server URL

[SAY-5]

Summary

startMeeting in server/plugin.go discarded the error from url.Parse and then read jURL.Hostname():

// Error check is done in configuration.IsValid()
jURL, _ := url.Parse(p.getConfiguration().GetJitsiURL())
...
claims.Subject = jURL.Hostname() // panics if jURL is nil

url.Parse(" https://...") returns (nil, err) because the leading space breaks scheme parsing, and IsValid() runs on config change, not per command. So an admin who pasted a URL with trailing or leading whitespace saw the plugin panic on the first /jitsi invocation (#281) and every subsequent command returned the opaque "plugin crashed, contact your administrator" message.

Fixes #281.

Change

• Parse the already-trimmed jitsiURL local variable (which is strings.TrimSpace(...)'d a few lines up) instead of the raw config value, so trimming actually reaches url.Parse.
• Don't discard the error: if parseErr != nil || jURL == nil, return "Jitsi plugin is misconfigured: invalid Jitsi Server URL" via errors.New (the already-imported github.com/pkg/errors).

Testing

• go build ./server/... passes.
• Manual regression: set JitsiURL to " https://meet.example.com " (spaces on both sides). Previously /jitsi panicked inside the plugin goroutine; now it returns the misconfigured-message error and the plugin keeps running.