fix(menu): change AI Hub submenu accelerator from H to A to resolve Help shortcut conflict

[AdamMagued]

Resolves #137

Summary of Changes

• In addon/globalPlugins/AIHub/plugin_mixins.py, updated the submenu insertion label from _("AI &Hub {addon_version}...") to _("&AI Hub {addon_version}...").
• Updated corresponding localization strings in addon/locale/uk/LC_MESSAGES/nvda.po and addon/locale/fr/LC_MESSAGES/nvda.po.
• Added unit tests in tests/test_menu_accelerator.py verifying that:
  • The submenu accelerator key is set to A (&A).
  • The submenu accelerator does not conflict with NVDAs built-in Help accelerator (H) or standard menu accelerators (Preferences, Tools, Help, Exit).
  • Localization files correctly define &AI Hub without obsolete AI &Hub entries.

Rationale

In the NVDA main menu, the standard Help menu item uses H as its accelerator key (&Help). Placing the ampersand before H in AI &Hub caused an accelerator key collision where pressing h conflicted between Help and AI Hub. Shifting the accelerator to A (&AI Hub) provides a unique top-level shortcut while freeing h for Help.