Remove legacy WPF app, flatten WinUI project to repo root, rename namespace

The repo carried two parallel UIs (WPF + WinUI) sharing Models/Services via
cross-directory Link includes. Now that WinUI is the only frontend, collapse
the structure so the WinUI project IS the repo root instead of a nested
sibling folder:

- Delete the WPF project entirely (App.xaml, MainWindow.xaml, csproj) and its
  bin/obj output
- Move AmiReel.WinUI/* up to the repo root (App, MainWindow, MainPage,
  DialogHelper, Assets, Package.appxmanifest, app.manifest, Properties,
  .github/instructions, AGENTS.md) via git mv, preserving history
- Rename AmiReel.WinUI.csproj -> AmiReel.csproj; regenerate the solution as
  AmiReel.slnx (the newer XML solution format) with a single project
- Rename namespace AmigaDB.VideoRenderer.{Models,Services} -> AmiReel.{...}
  and AmiReel_WinUI -> AmiReel across all files, including the embedded
  ffmpeg/ffprobe resource logical names in the csproj and ToolExtractor
- Models/ and Services/ no longer need the Link-based cross-directory
  <Compile Include>; they're picked up by the SDK's default globbing now
  that they live under the project directory
- Rename assets/ -> branding/ (source icon art) to avoid a case-insensitive
  collision with Assets/ (packaged tile art) once both sit at repo root
- Merge the two .gitignore files into one; track the PublishProfiles pubxml
  files instead of ignoring them (no secrets, and they keep publish
  reproducible across machines) as branding, gitignore, etc.
- Simplify publish-win-x64.ps1 (drop the -Target Wpf/WinUI switch, there's
  only one target now) and rewrite README.md to describe the single-project
  layout, build/run/publish commands, and file structure

Verified: dotnet build succeeds for both AmiReel.csproj and AmiReel.slnx, and
the built exe launches and renders identically to before the move.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -1,14 +1,55 @@
-bin/
-obj/
+# Build output
+[Bb]in/
+[Oo]bj/
+[Dd]ebug/
+[Rr]elease/
+publish/
+
+# IDE / tooling
 .vs/
 .vscode/
+_ReSharper*/
+*.DotSettings.user
+*.dotCover
+.idea/
+
+# MSBuild / user files
+*.user
+*.suo
+*.userosscache
+*.sln.docstates
 *.pdb
 *.cache
 *.tmp
 *.log
-*.userosscache
+*.binlog
+
+# Test results
 TestResults/
+[Tt]est[Rr]esult*/
+*.trx
+*.coverage
+*.coveragexml
+
+# NuGet
+*.nupkg
+*.snupkg
+*.nuget.props
+*.nuget.targets
+project.lock.json
+
+# MSIX packaging output
+AppPackages/
+BundleArtifacts/
+*.msix
+*.msixupload
+*.appx
+*.appxbundle
+*.appxupload
+
+# Local dev signing certificate
+devcert.pfx
+
+# Embedded third-party binaries (fetched separately, see ThirdParty/README.md)
 ThirdParty/ffmpeg.exe
 ThirdParty/ffprobe.exe
-*.user
-*.suo