🧪 Add tests for getUserInputPrefix helper

[tushuhei]

This PR adds unit tests for the getUserInputPrefix helper function in src/pv-app.ts. This function uses a regex to filter keyboard-inputtable characters (Latin letters, Hiragana, spaces, and long vowel marks).

The new tests cover:

• Standard Latin input.
• Hiragana input with spaces and long vowel marks (ー).
• Empty string edge case.
• Boundary cases where the function stops at disallowed characters (Numbers, Katakana, and Symbols).

Additionally, some minor linting and formatting issues were fixed to ensure a clean build.

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PR created automatically by Jules for task 4637760691737969135 started by @tushuhei

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