Capitalise I and its contractions in suggestions

[enaboapps]

Closes #909. Stacked on #916; part of the casing plan in #914.

What changed

• In model.rs, display treats any key that is "i" or starts with "i'" as the pronoun and shows it with a capital I: "I'm", "I'll", "I've", "I'd". Keys are already normalised to a straight apostrophe, and the typed apostrophe style is still applied afterwards, so a typed "i’" gives "I’ll".
• Applies to single words and to both words of a phrase.
• A unit test covers "i", "i'm", "i'll" with a curly apostrophe, and that "is" is untouched. The bundled-model test adds "Tomorrow " + "i'", which yields I'm, I'll, I'd and I've, with phrases "I'm going" and "I'll be".
• docs/word-prediction.md notes the rule.
• A review noted the docs overstated the rule: typed letters keep their case, so the sentence now says the capital applies to the completed letters.

Validation

Node 24.13.0, Rust 1.97.1, on this top of the stack:

• npm run lint, npm test (244 Vitest tests and 5 Node tests) and npm run build passed. No frontend files changed.
• cargo fmt --check and cargo clippy --all-targets -D warnings passed.
• cargo test passed: 567 tests.

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