feat: Add comprehensive NL-43/NL-53 Communication Guide and command references

- Introduced a new communication guide detailing protocol basics, transport modes, and a quick startup checklist.
- Added a detailed list of commands with their functions and usage for NL-43/NL-53 devices.
- Created a verified quick reference for command formats to prevent common mistakes.
- Implemented an improvements document outlining critical fixes, security enhancements, reliability upgrades, and code quality improvements for the SLMM project.
- Enhanced the frontend with a new button to retrieve all device settings, along with corresponding JavaScript functionality.
- Added a test script for the new settings retrieval API endpoint to demonstrate its usage and validate functionality.

docs/NL43_quickref.md

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+
+# ✅ NL-43 / NL-53 Command Format — Verified Quick Reference
+
+This cheat sheet lists confirmed working command formats for NL-43 and NL-53 devices, based on the official Rion Communication Guide (pages 30–96).
+
+---
+
+## 🧠 Protocol Basics
+
+- **Command format:** `Command,Param[CR][LF]` — no space after comma.
+- **Query format:** `Command?[CR][LF]`
+- **Line endings:** CR = `\r` (0x0D), LF = `\n` (0x0A), always use both.
+- **No `$` prefix** unless specifically required (e.g. system control).
+
+---
+
+## ✅ Confirmed Commands
+
+### 📏 Start/Stop Measurement
+| Action       | Command Sent              |
+|--------------|---------------------------|
+| Start        | `Measure,Start\r\n`     |
+| Stop         | `Measure,Stop\r\n`      |
+
+> **Important:** These must be exact — no space after comma, param is a capitalized string.
+
+---
+
+### 🕒 Set/Query Clock
+| Action       | Command Sent                                |
+|--------------|---------------------------------------------|
+| Set Time     | `Clock,2025,12,23 23:45:00\r\n`           |
+| Query Time   | `Clock?\r\n`                              |
+
+---
+
+### 📈 One-Shot Readout (DOD)
+| Action       | Command Sent |
+|--------------|--------------|
+| Get Snapshot | `DOD?\r\n` |
+
+Returns: comma-separated line with values like `R+70.2,91.1,88.0,...`
+
+---
+
+### 🔁 Streaming Output (DRD)
+| Action             | Command Sent |
+|--------------------|--------------|
+| Start DRD Output   | `DRD?\r\n` |
+| Stop DRD Output    | `\x1A` (SUB)|
+
+---
+
+### 🔧 Echo On/Off
+| Action     | Command Sent  |
+|------------|---------------|
+| Enable     | `Echo,On\r\n` |
+| Disable    | `Echo,Off\r\n` |
+| Query      | `Echo?\r\n` |
+
+---
+
+## ⚠️ Common Mistakes to Avoid
+- ❌ Don’t include a space after comma: `Measure, Start` → invalid.
+- ❌ Don’t use numeric params if the spec requires strings.
+- ❌ Don’t forget `\r\n` line ending — most commands won’t work without it.
+- ❌ Don’t send multiple commands at once — insert 1s delay between.
+
+---
+
+This file is safe for ingestion by agents or UI generators.