Add custom dictionary support

[human37]

Summary

Closes #34

Adds custom dictionary support so users can correct words/phrases Whisper consistently mishears. Two-layer approach:

• Layer 1 (prompt hint): Vocabulary words are passed to whisper-cli via --prompt to bias the decoder toward recognizing them
• Layer 2 (post-processing): Greedy sliding-window exact-match replacement on the transcript output, supporting both single-word and multi-word phrases

Users can manage dictionary entries via a new "Custom Dictionary..." settings window (NSTableView with editable columns), or by editing config.json directly:

{
  "customDictionary": [
    { "from": "nural", "to": "neural" },
    { "from": "chat gee pee tee", "to": "ChatGPT" }
  ]
}

Files changed

• Config.swift - Added DictionaryEntry struct and customDictionary field
• DictionaryPostProcessor.swift - Prompt building + sliding window replacement (new)
• Transcriber.swift - --prompt injection
• AppDelegate.swift - Wired into transcription pipeline
• DictionaryWindowController.swift - Settings window with NSTableView (new)
• StatusBarController.swift - Menu item + config reload integration

Test coverage

• 16 unit tests for DictionaryPostProcessor (single word, multi-word, punctuation, greedy matching, edge cases)
• 3 config decoding tests