fix(lesson): handle uppercase letters in word filtering

[Dronakurl]

PR Message Template: Uppercase Words Fix

Summary

Fix case-sensitivity bug in dictionary filtering that prevented uppercase words from being used in guided lessons.

Related Issue

Fixes #555

Current State

When "Prefer natural words" is enabled, words with uppercase first letters are filtered out because the dictionary uses case-sensitive character matching. The German alphabet contains only lowercase letters, so words like "Hexe", "Mexiko", "Loyalität" are excluded from lessons.

This results in:

• Only 7 out of 23 words with 'x' being available (30%)
• Only 9 out of 19 words with 'y' being available (47%)
• Pseudo-words like "loyalig" and "mexist" being generated for rare letters

Proposed State

Convert characters to lowercase before checking against the codePoints set, allowing all words in the dictionary to be used regardless of capitalization.

Changes

Modified Files

• packages/keybr-lesson/lib/dictionary.ts
  • Modified filterWordList() to convert characters to lowercase before filtering
  • Modified Word.matches() to convert characters to lowercase before matching

Code Changes

// filterWordList - before:
words.filter((word) =>
  [...toCodePoints(word)].every((codePoint) => codePoints.has(codePoint)),
)

// filterWordList - after:
words.filter((word) =>
  [...toCodePoints(word)].every((codePoint) => {
    const lower = String.fromCodePoint(codePoint).toLowerCase().codePointAt(0)!;
    return codePoints.has(lower);
  }),
)

Same pattern applied to Word.matches().

Impact

Letter	Before	After	Change
x	7 words (30%)	23 words (100%)	+227%
y	9 words (47%)	19 words (100%)	+111%

Testing

• Existing tests should pass (no regressions expected)
• Manual testing with German language focusing on 'x' and 'y' letters
• Verified that real words like "Hexe", "Mexiko", "Hobby" now appear in lessons

Related

• Only limited and biased selection of words with Natural Words #555 - Issue describing the problem
• uppercase words for german wordlist #530 - Related issue about uppercase words (closed as user confusion)
• add more words containing q and j for german #268 - PR that added more q/j words (similar symptom, different root cause)

Limitations

• Short words (< 3 characters) are still filtered out by design (see guided.ts:31)
• ALL CAPS words (acronyms) would pass, but none exist in the German word list
• No changes to the phonetic model - it continues to generate lowercase pseudo-words as fallback

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
packages/keybr-lesson/lib/dictionary.ts	40% smaller
packages/keybr-color/lib/convert-xyz.test.ts	24% smaller
build.sh	Unsupported file format
packages/keybr-lesson/lib/dictionary.test.ts	0% smaller

[trapicki]

Please pull this request.

I tried it, and it really is a huge improvement for German training! Not only with the rare letters, but also with all the other letters it shows so much more words that where hidden. German has inherently many words with initial uppercase, so these words and having them shown in uppercase is exactly doing what keybr has as a unique feature: practice words and letter combinations like they occur in daily writing.