feat: add "fetch all repositories" action in workspace menu

[JC-Chung]

What

Add a "Fetch All Repositories" menu item to the workspace dropdown menu,
allowing users to fetch all open repositories in the active workspace with
a single action.

Why

When working with multiple repositories in a workspace, users currently have
to switch to each tab and trigger fetch individually (e.g., Ctrl+Down).
This action automates that process without requiring tab switching.

Changes

• src/ViewModels/Repository.cs: Add FetchAllRemotesAsync() that directly
  runs Commands.Fetch for all remotes (similar to auto-fetch behavior),
  showing the IsAutoFetching overlay instead of the Fetch.axaml popup.
  Repositories with no remotes or an active index.lock are skipped silently.
• src/ViewModels/Launcher.cs: Add FetchAllRepositoriesAsync() that
  iterates over a snapshot of Pages (to avoid collection-modified exceptions
  during async enumeration) and calls FetchAllRemotesAsync() on each open
  repository. Sends a completion notification to the active page when done.
• src/Views/Launcher.axaml.cs: Add menu item with Icons.Fetch icon in
  OnOpenWorkspaceMenu after the separator, enabled only when at least one
  repository is open.
• src/Resources/Locales/: Add Text.Workspace.FetchAllRepositories key to
  en_US.axaml, zh_CN.axaml, and zh_TW.axaml.