add EMA smoothing to CPU and temperature widgets

[neoden]

• EMA smoothing for CPU and Temperature widgets to reduce flickering
• Rename average_seconds → smoothing_interval_seconds (breaking change)
• Warn about unknown config keys

[Copilot]

Pull request overview

This PR adds Exponential Moving Average (EMA) smoothing to CPU and Temperature widgets to reduce flickering in the status bar display. It also introduces a breaking configuration change by renaming average_seconds to smoothing_interval_seconds and adds warnings for unknown configuration keys to help users migrate.

Changes:

• Implemented new EMA utility package with comprehensive tests for exponential moving average smoothing
• Refactored CPU widget to use EMA instead of simple averaging, with updated tests
• Added EMA smoothing to Temperature widget
• Renamed config parameter from average_seconds to smoothing_interval_seconds (breaking change)
• Added detection and warnings for unknown/deprecated config keys
• Updated default smoothing interval from 5 to 3 seconds

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
util/ema.go	New EMA implementation with period-based alpha calculation and readiness tracking
util/ema_test.go	Comprehensive test suite for EMA functionality including edge cases
widgets/cpu.go	Refactored to use EMA smoothing instead of history-based averaging, added math.Round for display
widgets/cpu_test.go	Updated tests to verify EMA behavior instead of history management
widgets/temperature.go	Added EMA smoothing to temperature sensors with default interval handling
config/config.go	Renamed field and added unknown key detection with warnings to stderr
config/config_test.go	Updated tests for renamed field and added test for unknown key warnings
config.example.toml	Updated documentation with new parameter names and better descriptions

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