Add Airable streaming support: Radio, Podcasts, and UPnP (#20)

* airable streaming; radio, podcasts and UPnP servere

* Update readme and changelog

* Bumped the year

* Updates

* Event tracking

* Refactor: Add command factories to reduce code duplication

- Add MakeCategoryCommand factory for radio category commands
- Add MakePodcastCategoryCommand factory for podcast category commands
- Add MakeStationCompletion and MakePodcastCompletion factories
- Add HandlePickerResult shared result handler
- Replace 5 radio commands and 3 podcast commands with factory calls
- Net reduction of ~930 lines of duplicated code

* Cleanup: Remove unused code and consolidate duplicates

Removed unused code:
- cacheConfigSettings variable (config_cache.go)
- loadingMsg type (radio.go)
- upnpContainerStyle, upnpAudioStyle variables (upnp.go)
- FilterItemsByType function (completion_helpers.go)
- DiscoverSpeakersLegacy function (kefw2/discover.go)
- RadioStation, Podcast, PodcastEpisode, MediaServer, UPnPTrack type structs
- GetRadioHistory, GetRadioRecommendations methods (kefw2/airable_radio.go)

Consolidated duplicates:
- parseDisplayPath: merged radio and podcast versions into airable.go
- findItemByName: replaced findStationByName and findPodcastByName

Net reduction: 119 lines (71 added, 190 removed)

* Refactor: Unify radio/podcast Airable services and add CLI error helpers

- Create airable_service.go with unified service abstraction for radio/podcast
- Reduce airable_radio.go from 622 to 221 lines (-64%)
- Reduce airable_podcast.go from 537 to 185 lines (-66%)
- Add exitOnError/exitWithError helpers to reduce CLI boilerplate
- Apply error helpers across 10 CLI command files
- Consolidate findItemByName with optional type filtering
- Remove unused pp dependency and clean up examples
- Remove commented scaffolding code from root.go

* refactor(cli): replace verbose error handling with exitOnError/exitWithError helpers

- Convert os.Exit(1) patterns across 10 CLI command files
- Use centralized exitOnError() for error checking with message
- Use exitWithError() for unconditional error exits
- Remove 289 lines of boilerplate error handling code
- Build and tests pass

Author: hilli