feat: cursor navigation, key sounds, dictionary tooling, key security

Batch of in-progress app work from the working tree.

- feat(keyboard): CursorNavigation + CursorDragPad for caret movement;
  KeyboardSoundFeedback for system key click sounds
- feat(dictionary): DictionaryAliasGenerator + PersonalDictionaryEntrySheet;
  TranscriptPostProcessor quality gate; retire DictionaryLearner
- feat(ui): TabBarVisibility handling; drop PageHeaderRow /
  PageHeaderConfirmButton; refresh views and localizable strings
- fix(security): move the hardcoded DeepSeek key out of
  PreconfiguredKeys.swift into a gitignored PreconfiguredKeys.local.swift
  (seeded from .example by generate-xcodeproj.sh)
- docs(agents): add Conventional Commits versioning + bilingual changelog rules
- chore(gitignore): ignore PreconfiguredKeys.local.swift, .cache/, pycache

Custom language model / lexicon work stays on
feature/custom-language-model-asr. Changelog bullets added under
[Unreleased]; no version bump.

.gitignore

@@ -45,6 +45,7 @@ fastlane/test_output
 # Local config (API keys, signing, etc.)
 *.local
 Signing.local.xcconfig
+PreconfiguredKeys.local.swift
 .env
 .env.*
 
@@ -53,3 +54,10 @@ Signing.local.xcconfig
 
 # Local DerivedData (when using -derivedDataPath in repo)
 .derivedData/
+
+# Lexicon build cache (downloaded SogouPopularDict TSV)
+.cache/
+
+# Python bytecode
+__pycache__/
+*.pyc