Configurable log verbosity — replace debug toggle with log level dropdown

[nitrobass24]

Summary

Replace the binary debug toggle (on/off) with a configurable log level selector supporting all standard Python log levels: DEBUG, INFO, WARNING, ERROR, CRITICAL.

Motivation

The current logging config is a boolean — config.general.debug toggles between INFO and DEBUG. Users have no way to set WARNING/ERROR/CRITICAL levels to reduce log noise in production, or to fine-tune verbosity.

Current State

• config.general.debug (bool) — True = DEBUG, False = INFO
• CLI --debug flag overrides the config value
• Logging supports standard and JSON formats (already configurable)
• Multiprocess logging already works via QueueHandler

Implementation

Config change (clean break)

• Remove debug property from Config.General
• Add log_level string property (values: DEBUG, INFO, WARNING, ERROR, CRITICAL)
• Default: INFO
• The config parser already silently ignores unknown keys, so existing debug = True lines in old config files won't crash on upgrade — they'll just be ignored, and log_level defaults to INFO

Python backend

• Update Seedsync._create_logger() to use config.general.log_level instead of the debug boolean
• Update --debug CLI flag to set log_level = DEBUG (backward compat for CLI users)
• Update multiprocess log level propagation

Angular UI

• Replace the "Enable Debug" checkbox in the Logging section with a "Log Level" dropdown
• Choices: DEBUG, INFO, WARNING, ERROR, CRITICAL
• Update Config.General interface and DEFAULT_GENERAL

Config serialization

• Update serialize_config.py and tests

Acceptance Criteria

• debug bool removed from config, replaced with log_level string
• Default is INFO; old configs with debug key don't crash on startup
• UI shows dropdown with all 5 log levels in Logging section
• --debug CLI flag still works (sets log_level to DEBUG)
• All log levels work correctly (verified with each level)
• Tests updated