Add sign-in page with FreshRSS and Instapaper authentication options

[Copilot]

Sign-in page with 65/35 split layout — abstract image left, auth buttons right. Strings localized via .resw.

Changes

• SignInPage.xaml — Two-column grid (65*/35*) with abstract background image on left, vertically centered sign-in panel on right. FreshRSS uses AccentButtonStyle, Instapaper uses default. All strings bound via x:Uid.
• Strings/en-us/Resources.resw — l10n entries: SignIn_Title.Text, SignIn_Subtitle.Text, SignIn_FreshRSS.Content, SignIn_Instapaper.Content
• Assets/SignInBackground.png — Generated abstract image (deep blue-teal gradient, geometric shapes, flowing curves, bookmark icon)
• MainWindow.xaml — Replaced test ListView with Frame for page navigation
• MainWindow.xaml.cs — Navigates to SignInPage on load via RootFrame.Navigate(typeof(SignInPage))
• .csproj — Added SignInBackground.png as Content (.resw files are auto-included by the SDK as PRIResource)

Preview

Abstract background image:

Full page mockup:

Button click handlers are stubbed — actual auth flows to be implemented separately.

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[Kumara-Krishnan]

@copilot the build check is failing

[Copilot]

@copilot the build check is failing

The build was failing due to NETSDK1022: Duplicate 'PRIResource' items — the .NET SDK auto-includes .resw files from the project directory, so the explicit PRIResource glob I added caused a conflict. Removed it in 1764b8b.