Send the real Shift state to the prediction worker

[enaboapps]

Closes #907. First of the casing changes tracked in #914.

What changed

• The private worker request carries Shift::{Off, Once, Locked} instead of a boolean. This is the in-process pipe message only; no persisted schema or protocol changes.
• The keyboard reports its Shift modifier as that enum. An automatic Shift after a sentence end is still reported as off, because the worker capitalises there itself.
• A new cased function in worker.rs decides the suffix case in one place:
  • Caps or Shift locked uppercase the whole completion, and together they cancel, as on typed letters.
  • Before any letter of the word is typed, Shift once flips only the first character, the way it would flip the next typed letter. So "Water" normally, and "wATER" under Caps. A sentence start capitalises the first letter without any modifier.
  • After typed letters, a pending Shift once is left for the next letter and does not change the completion.
• The old test that asserted Shift once gives "WATER" now expects "Water", and a new test covers every Shift and Caps combination before and after typing.
• docs/word-prediction.md describes the rule.
• A follow-up commit keeps a name's capital in an unmodified suggestion: only an explicit Shift once flip lowers a first letter. The fake model gained "WhatsApp" and the test checks it survives.
• A re-review found the doc sentence "modifiers still mirror typing at a sentence start" overclaimed, since Shift locked with Caps capitalises there where typing would not. The sentence now says only that Shift once under Caps gives a lowercase first letter, and a test covers Caps, Locked with Caps and Once with Caps at a sentence start.

Validation

Node 24.13.0, Rust 1.97.1:

• 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: 566 tests.

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