memtemp-plus: KeyError crashes when config options missing

[CoderFX]

Bug

memtemp-plus.py crashes with KeyError when config options are not set in config.toml:

1. self.options['scale'] in cpu_temp() — crashes if scale not configured
2. self.options['orientation'] in on_ui_setup, on_unload, on_ui_update — crashes if orientation not configured
3. self.fields not initialized in __init__ — AttributeError if on_unload called before on_ui_setup
4. cpu_freq() reads sysfs file without error handling — crashes if file doesn't exist

Steps to Reproduce

Add [main.plugins.memtemp-plus] with enabled = true but without setting scale or orientation.

Fix

PRs incoming with .get() defaults and error handling.