fix: show receipt thumbnail in expense detail view

[daveharmswebdev]

Summary

• Replace "Receipt attached" text with actual receipt thumbnail image in expense detail view
• Shows thumbnail in both view mode and edit mode receipt cards
• Fetches receipt viewUrl reactively via Angular effect() when expense has a receiptId
• PDF receipts show a document icon; image receipts show the actual thumbnail
• Clicking the thumbnail opens the existing lightbox dialog

Test plan

• Navigate to expense with linked receipt — thumbnail displays instead of "Receipt attached" text
• Click thumbnail — lightbox opens
• Expense with PDF receipt — shows document icon instead of image
• Expense without receipt — shows "No receipt linked" (unchanged)
• Edit mode with receipt — thumbnail shown alongside View/Unlink buttons
• All 2420 frontend tests pass

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