chore: fix typos, remove stale comment, and update codecov-action to v4

- Remove misleading "For Debuging" comment in sys_session.go
- Fix "offical" → "official" typo in handlers_session.go
- Update codecov-action@v3 → @v4 in CI workflow
- Consolidate technical-debt.md with completed items and updated TOC

server/channelserver/handlers_session.go

@@ -391,7 +391,7 @@ func handleMsgSysIssueLogkey(s *Session, p mhfpacket.MHFPacket) {
 		return
 	}
 
-	// TODO(Andoryuuta): In the offical client, the log key index is off by one,
+	// TODO(Andoryuuta): In the official client, the log key index is off by one,
 	// cutting off the last byte in _most uses_. Find and document these accordingly.
 	s.Lock()
 	s.logKey = logKey

server/channelserver/sys_session.go

@@ -70,7 +70,6 @@ type Session struct {
 	// Contains the mail list that maps accumulated indexes to mail IDs
 	mailList []int
 
-	// For Debuging
 	Name     string
 	closed   atomic.Bool
 	ackStart map[uint32]time.Time