Refresh README for current Dictate features

[MyButtermilk]

Summary

Refreshes the README so it matches the current Dictate Keyboard feature set and reads more like a concise product page than a historical project note.

What changed:

• Reworked the hero copy around the current value proposition: AI dictation keyboard, realtime transcription, offline STT, AI rewriting, Wear OS and full FlorisBoard typing.
• Kept the existing logo, Google Play CTA, PayPal CTA, demo GIF/video link and screenshot grid.
• Added a clear "Why Dictate is different" comparison table.
• Added GFM admonitions for the FlorisBoard rebuild note, provider/privacy expectations and upgrade tip.
• Expanded feature coverage for recent work: realtime transcription, on-device Whisper/Parakeet, file transcription, floating button, Wear OS, classic voice-first layout, prompt chips, live prompts, per-prompt reasoning, custom words, history, stats, resend/recovery, proxy/certificate options and backup-aware data.
• Split supported providers into speech-to-text and AI rewriting groups so users quickly understand what they can connect.
• Removed dedicated Contributing / License-style README sections because those belong in the repo's dedicated files.

Why this helps

The old README was partly accurate but undersold several major additions that have landed recently. New users should be able to understand, in the first screen and first few sections, that Dictate is not just another voice note app or a basic Whisper wrapper. It is a full Android keyboard that can also float above other keyboards, run on Wear OS, work offline, use multiple providers and post-process text with reusable prompts.

The new structure keeps the README concise while making the conversion path clearer:

• understand the product quickly,
• see the demo and screenshots,
• understand the differentiators,
• scan the full feature set,
• verify provider support,
• install or support development.

Validation

• Checked git diff --check successfully.
• Verified all local README image/demo references exist.
• Documentation-only change; no Gradle tests were run.