Add a full stop key to the Simple QWERTY letters page

[enaboapps]

Closes #1048

What changed

• Simple QWERTY Letters page (src-tauri/src/scan_keyboard.rs): the bottom row is now Shift z x c v b n m . Space. The new key is Key::Character('.', '.'), so Shift, Caps and the automatic sentence Shift all still type . and never >.
• No change was needed in smart_mark. Every supported mark on the Letters page already goes through smart punctuation, so this key is handled like the Numbers-page ./> prose key in Simple QWERTY. It replaces an owned space, types . , sets sentence-start capitalisation for the next letter and gives prediction the sentence-start context. The separate numeric-pad . on Numbers is still a literal decimal point.
• The QWERTY and Common letters layouts are unchanged. The existing test that the other layouts' Letters rows are unchanged still passes.
• Settings copy (ScannerPreferences.tsx, ScanningSection.tsx) now mentions the full stop on Letters.

Tests

• Updated simple_qwerty_has_three_letter_rows_and_keeps_removed_keys_reachable for the new bottom row.
• Added simple_qwerty_letters_full_stop_stays_a_full_stop_and_starts_a_sentence. It checks that the key gives Output::Punctuation('.') when typed plainly, at a sentence start with the automatic Shift, with a manual Shift and with Caps. It also checks that the next letter is capitalised. The executor side (. typed through the fake injector, with the owned-space Backspace) is already covered by smart_punctuation_edits_only_an_owned_space_and_cleans_up_failures.
• The panel-fit and bounds tests (layouts_and_predictions_keep_bounds_independent_of_text across all three panel sizes) pass. Because Space is wider than other keys, the bottom row is now the widest row: the panel's preferred width grows from about 921 to 997 px, and the panel scales down to fit narrow screens as before.

Validation (Windows, Node 24.13.0)

• npm run lint: pass

• npm test: 13 files, 295 tests passed

• npm run build: pass

• cargo fmt --check: pass

• cargo clippy --all-targets -- -D warnings: pass

• cargo test: 772 passed, 5 ignored; config 7 passed

• npm run prediction-model was not run because of a local EPERM rename lock. Instead I copied the gitignored src-tauri/resources/prediction-model, src-tauri/resources/prediction-neural and src-tauri/binaries from the main checkout.

• Independent review of c9e2383 reported no actionable findings. It confirmed the key always types ., the Numbers-page decimal point is unaffected, and the other layouts are unchanged.

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