feat: 💄 modernize the ux

[amd989]

Summary

• Complete UX redesign: replaced the old form-based layout (TextBoxes, ComboBoxes, labels) with a card-based design featuring two side-by-side cards (Link Location and Target) connected by an arrow
• Pill-style radio buttons: replaced ComboBox dropdowns with pill toggles for type selection (Folder/File) and link type (Symbolic link, Junction, Hard link)
• Themed in-window dialogs: replaced all Win32 MessageBox.Show calls with MahApps Metro ShowMessageAsync dialogs that match the app theme, with animations disabled for snappy feedback
• Edge case hardening: double-click prevention on Create Link button, success fallback message when mklink produces no stdout, quote validation in paths, string interpolation cleanup

Test plan

• Launch the app and verify the card-based layout renders correctly
• Test Folder and File pill toggles -- Junction should only appear in Folder mode, Hard link only in File mode
• Browse for a link location and target using the icon buttons
• Drag and drop a folder onto the Target card -- verify it switches to Folder mode
• Drag and drop a file onto the Target card -- verify it switches to File mode
• Type a long path and verify the caret stays visible (horizontal scroll)
• Hover over text fields -- verify the underline appears
• Click a text field -- verify the underline turns accent-colored
• Create a symbolic link and verify the themed success dialog appears
• Try creating with empty fields -- verify the themed error dialog appears
• Switch Windows between light and dark theme -- verify all elements update, including the Target card's folder/file name
• Click the About (info) button in the footer
• Rapidly double-click Create Link -- verify only one operation runs