chore: Clean up README and fix German keyboard default

[Hugo0]

• Replace stale 65-line language table with concise summary
• Remove completed TODO items (keyboards, profanity filtering)
• Remove ngrok and Docker sections
• Add wordfreq to data sources
• Delete stale n_words.png chart
• Default German keyboard to Simple QWERTZ (diacritic_map handles umlauts)

Summary by CodeRabbit

New Features

• Added Simple QWERTZ keyboard layout option for German users.

Documentation

• Updated language coverage documentation now displays support for 65 languages with details on available supplement and frequency-curated word lists.
• Enhanced language documentation structure with improved data source references and organization.

[coderabbitai]

Caution

Review failed

The pull request is closed.

📝 Walkthrough

Walkthrough

The pull request updates project documentation by replacing the mobile testing section with language coverage details (65 supported languages) and modifies the German keyboard configuration by adding a new "simple_qwertz" layout and setting it as the default.

Changes

Cohort / File(s)	Summary
Documentation README.md	Replaces "Testing on mobile with ngrok" section with "Language Coverage" section listing 65 languages; consolidates Docker and language status information; updates TODO list; extends Credits/Data Sources with wordfreq reference.
Keyboard Configuration webapp/data/languages/de/de_keyboard.json	Adds new "simple_qwertz" keyboard layout and updates the default layout reference from "german_qwertz" to "simple_qwertz".

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• Refine Polish dictionaries and UI translations #115: Updates Polish language data and SOURCES.md similar to how this PR restructures language documentation and updates German keyboard data.

Poem

🐰 A language-friendly hop we take,
From mobile tests to coverage's sake,
QWERTZ keys now simply arranged,
Sixty-five tongues, beautifully changed! ✨

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/frequency-word-lists

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