Ignore *.csproj.user files in server project

[DKuleshov]

Problem

server/src/Dotbot.Server/Dotbot.Server.csproj.user is user-specific IDE state (Visual Studio per-user project settings) and currently shows up as an untracked file in git status. It should never be committed.

Proposal

Add server/**/*.csproj.user to the root .gitignore under the existing Server (DotbotServer C# app) section (around line 38-53).

Acceptance

• git status no longer lists server/src/Dotbot.Server/Dotbot.Server.csproj.user
• Any other *.csproj.user under server/ is also ignored