feature: add "Commit & Force Push" action when amending a commit

[sdpfigueiredo]

Closes#2575

Description

Implements the requested Commit & Force Push action in the Local Changes panel, making the amend workflow consistent with the existing Commit & Push workflow.

• The Commit & Push button is now a SplitButton with a dropdown offering two actions:
  • Commit & Push — unchanged behavior
  • Commit & Force Push
• Selecting Commit & Force Push first shows a confirmation dialog warning that the remote branch history will be rewritten.
• After confirmation, the commit is created and pushed via the existing push machinery with --force-with-lease, so concurrent remote changes by other users are never overwritten blindly (per the issue's safety requirement).

Notes

• The dropdown is visible whenever the repository has remotes, so the action works both with and without the Amend checkbox — it matters most when Amend is enabled, since rewriting an already-pushed commit requires a force push.
• New localization keys were added to en_US.axaml; all other locales fall back to it via ResourceInclude.

Testing

• dotnet build src/SourceGit.csproj -c Debug — 0 warnings, 0 errors
• dotnet publish src/SourceGit.csproj -c Release -r win-x64 — Native AOT publish succeeds
• Smoke-tested the published Windows x64 build

Screenshot

[love-linger]

Force Push is a dangerous operation. SourceGit requires user to manually push with --force enabled.