fix: handle closed connection errors gracefully

[fclairamb]

Summary

When a client disconnects immediately after connecting (before sending any commands), the server would log multiple error messages that appear alarming but are actually normal FTP client behavior.

Problem

Some FTP clients (like KDE Dolphin, certain Go clients) may:

• Open a probe connection and immediately close it
• Disconnect before the server can read the first command

This resulted in error logs like:

level=error err="set tcp ...: use of closed network connection" event="Network error"
level=error err="read tcp ...: use of closed network connection" event="Network error"

Solution

Detect "use of closed network connection" and "connection reset by peer" errors and handle them as normal disconnects (debug level) instead of errors.

Changes in two code paths:

1. readCommand(): Check if SetDeadline fails due to closed connection and return early
2. handleCommandsStreamError(): Treat closed connection errors as normal disconnects

Before

level=error ... err="set tcp ... use of closed network connection" event="Network error"
level=error ... err="read tcp ... use of closed network connection" event="Network error"

After

level=debug ... event="Client disconnected before first command"

(Only visible when debug logging is enabled)

Test plan

• All existing tests pass
• go build ./... succeeds
• Manual test with FTP client that exhibits this behavior

Fixes #710

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.73%. Comparing base (153f345) to head (2ad6aa6).

Files with missing lines	Patch %	Lines
client_handler.go	50.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #602      +/-   ##
==========================================
- Coverage   87.14%   86.73%   -0.41%     
==========================================
  Files          12       12              
  Lines        1828     1840      +12     
==========================================
+ Hits         1593     1596       +3     
- Misses        157      164       +7     
- Partials       78       80       +2     

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