feat(chat): local hints for conversations

[MaxDac]

Description

Each conversation in "Conversations on this device" can now have a short private hint saying who is speaking. Hints are stored only locally.

• Storage: hints live in PeerMeta.hint, inside the encrypted Signal state under noBackupFilesDir. They are never sent to the server, push payloads or notifications. This PR makes no API change and does not bump api_version.
• Normalization: whitespace is trimmed and collapsed, the length is limited to 40 code points, and a blank hint is removed.
• Lifecycle: "Clear history" keeps hints. Signing out (clearPeerHints) and "Forget device" (wipe) drop them.
• UI:
  • Long-press a conversation on the home screen, or open the chat overflow menu and choose "Add hint" / "Edit hint", to open HintDialog.
  • Each time the dialog opens, it shows an inline advice not to write the other person's name.
  • The field uses NoPersonalizedLearning.
  • The hint appears under the conversation name and in the chat header.
• Strings: added in all five locales.
• Docs: updated docs/ARCHITECTURE.md.

The matching website change is MaxDac/Privee#177. The two PRs don't depend on each other.

Validation:

• ./gradlew test :app:lintDebug :app:assembleDebug: passed, including after merging main.
• New tests:
  • SignalClientTest: normalization, persistence across "Clear history" and a restart, clearPeerHints, the code-point limit, and decoding old state that has no hint.
  • RecentsTest: peerId and hint in the list.
  • HintTest (Robolectric):
    • the advice is always shown;
    • Save normalizes the text;
    • Remove saves null;
    • a long-press opens the editor;
    • the chat header shows the hint.

Checklist

• No secrets committed (API keys, endpoints, tokens, .jks, .env)
• :core:* modules contain no android.* / androidx.* imports
• Tests pass (./gradlew test)
• Commit messages follow Conventional Commits