feat: redesign WPF UI with Fluent system

[devin-ai-integration]

Description

Implemented a comprehensive Fluent-inspired WPF UI refresh across the app in one unified PR to avoid a mixed old/new experience.

Key changes:

• Added a shared FluentDesign.xaml resource dictionary with design tokens for spacing, radius, page layout, cards, toolbars, list rows, dialogs, empty states, search boxes, combo boxes, toggles, and subtle short fade transitions.
• Applied the shared system consistently across shell/navigation, dashboard, settings, feature pages, optimization category pages, bloatware, disk cleanup, startup manager, scheduled tasks, and dialogs.
• Cleaned up page/header/toolbar/list density, replacing ad-hoc spacing and corner radii with reusable Fluent-inspired patterns while keeping optimizerDuck yellow/duck identity.
• Simplified shell title bar actions into icon-only command buttons to reduce noise.
• Improved dialog layout consistency and processing dialog hierarchy.
• Optimized GitHub source lookup caching in OptimizationCategoryViewModel to avoid duplicate concurrent source fetches.

Type of Change

• 🐛 Bug fix (fix: - non-breaking change which fixes an issue)
• ⚡ New Optimization/Feature (feat: - adds a new optimization or feature)
• ✨ New App Feature (feat: - adds UI, services, or core app functionality)
• 🛠️ Refactoring (refactor: - code restructure that neither fixes a bug nor adds a feature)
• 📝 Documentation update (docs:)
• 🌐 Translation/Localization update (i18n: or chore:)

Screenshots or Video (if applicable)

Not captured locally because this WPF app requires a Windows desktop/runtime; validation was performed via build on Linux with Windows targeting enabled and CI will run on windows-latest.

Additional Context

Validation performed locally:

• dotnet restore optimizerDuck.slnx -p:EnableWindowsTargeting=true
• dotnet build optimizerDuck.slnx --configuration Release --no-restore -p:EnableWindowsTargeting=true
• dotnet format optimizerDuck.slnx --verify-no-changes --include optimizerDuck/UI/ViewModels/Optimizer/OptimizationCategoryViewModel.cs --no-restore
• XML parse check for all XAML files
• git diff --check

dotnet test optimizerDuck.Test/optimizerDuck.Test.csproj --configuration Release --no-build -p:EnableWindowsTargeting=true cannot execute on this Linux VM because Microsoft.WindowsDesktop.App is Windows-only. CI should run tests on windows-latest per the workflow.

Link to Devin session: https://app.devin.ai/sessions/bf7d4a16910547ed9edd63891a20eeaa
Requested by: @itsfatduck

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring