Add apartment comparison feature (frontend)

[NigelTatem]

Summary

• Adds a new Comparison Page (/compare) that lets users compare up to 4 apartments side-by-side across price, rating, location, amenities, and more.
• Includes an Add Apartment Modal with two tabs - "Saved Properties" (grid of saved apartments) and "Search Properties" (debounced search with result cards).
• Adds a "Compare" button on ApartmentPage so users can add the current apartment to the comparison from its detail page.
• New components: ComparisonPage, AddApartmentModal, SearchResultCard, SavedAptCard, AmenityIcon.

Files Changed

File	Description
frontend/src/pages/ComparisonPage.tsx	Main comparison page with 4-slot layout, sticky row labels, horizontascroll
frontend/src/components/Comparison/AddApartmentModal.tsx	Modal with Saved/Search tabs for adding apartments to compare
frontend/src/components/Comparison/SearchResultCard.tsx	Search result card with photo, metadata, rating, selected state
frontend/src/components/Comparison/SavedAptCard.tsx	Saved property card for the grid view in the modal
frontend/src/components/Comparison/AmenityIcon.tsx	Renders check/X/dash icons for amenity availability
frontend/src/App.tsx	Added /compare route
frontend/src/pages/ApartmentPage.tsx	Added "Compare" button linking to comparison page
common/types/db-types.ts	Added Tag and TagWithId types

What's Done (Frontend)

• 4-slot comparison layout matching Figma designs
• Empty state with "Add an Apartment" placeholder cards
• Sticky row labels that stay visible during horizontal scroll
• Add Apartment modal with Saved Properties tab (grid layout)
• Add Apartment modal with Search Properties tab (debounced search)
• Search result cards with selected/unselected states
• Remove apartment from comparison (X button)
• "Compare" button on individual apartment pages
• URL sync via ?ids= query parameter
• Amenity icons (check / X / dash for unknown)

Still Needs To Be Done

Backend

• Room size field - not currently in the data model; comparison page shows "—" placeholder
• Year built field - not in the data model; shows "—" placeholder
• Amenity boolean fields (laundry, gym, parking, pool, pet-friendly) - not in the data model; all show dash icons currently
• Saved properties endpoint - modal's "Saved Properties" tab needs an API to fetch the user's saved/favorited apartments

Frontend (follow-up)

• Wire up "Saved Properties" tab to real saved-apartments API once backend is ready
• Populate amenity rows with real data once backend fields exist
• Add room size and year built once backend provides them
• Mobilelayout adjustments
• Unit tests for comparison components

Test Plan

• Navigate to /compare - empty state renders 4 placeholder cards
• Click "Add an Apartment" - modal opens with Search tab
• Type in search - results appear after debounce
• Select a result and confirm - apartment populates in the slot
• Add up to 4 apartments - all slots fill correctly
• Click X on an apartment - slot returns to empty state
• Horizontal scroll - row labels stay pinned on the left

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