Keep the model's capital on proper nouns mid-sentence

[enaboapps]

Closes #908. Stacked on #915; part of the casing plan in #914.

What changed

• model.rs tracks, for each finished word, how much of its probability mass comes from spellings that capitalise the last word. display now takes the finished entry rather than just its best spelling.
• Mid-sentence, a word keeps its capital when the capitalised spellings carry at least nine tenths of its mass. The model only prefers that for a proper noun, so "London" and "Monday" come through while a rare word whose capital is a coin toss, like "monkey", stays lowercase. Mixed-case names such as "WhatsApp" keep working as before.
• At a sentence start the model capitalises everything, so the lowercase key wins there and the worker's sentence rule decides, as before.
• The second word of a phrase follows the same mid-sentence rule. Sentence start is judged from the context the model saw: empty, or ending in a full stop, exclamation or question mark.
• Bundled-model cases for "I live in lon" and "See you on mon" assert London and Monday. The fake-scorer test that expected a capital-only spelling to be lowercased mid-sentence now expects it kept.
• docs/word-prediction.md describes the rule.
• A review asked for the threshold itself to be tested: the fake-scorer test now checks a share of 0.89 stays lowercase and 0.94 keeps the capital. It also noted the model occasionally capitalises a rare word such as Ketchup; the docs say so instead of claiming only proper nouns.

Sample output at the chosen threshold:

Context	Words
"I live in " + "lon"	London, Long, Longmont, Longview, Longwood
"See you on " + "mon"	Monday, Mondays, Mon, Monday's, monkey
"Can you send a " + "wh"	white, WhatsApp, whole, wheelchair, wheat

Validation

Node 24.13.0, Rust 1.97.1:

• cargo fmt --check and cargo clippy --all-targets -D warnings passed.
• cargo test prediction suite passed, including the bundled-model test. No frontend files changed; the full frontend suite runs on the top of the stack.

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