Fix bugs in init and config loading

[floatplane]

• fix a bug introduced in #29: I've now learned why the mysterious VERY IMPORTANT 2 second delay at startup exists
• fix a bug introduced in #31: to maintain backwards compatibility*, we need to load all config values as JSON
• fix ambiguity around how to interpret min/max temperature when the user changes the temperature unit
• move more strings into flash
• remove some unused globals

*I may decide to break backwards compatibility anyway