feat(site): platform-first redesign with web platform support

[TimPietruskyRunPod]

Summary

• Renames OsPlatform to Platform throughout, adds 'web' as a new platform variant
• Moves platform selector out of model filters into its own top-level section with a grid layout
• Adds per-platform draft state persistence (localStorage) so switching platforms remembers your selections
• Improves selected model card styling with accent color backgrounds and borders
• Adds resolveModelPlatforms() for flexible model-to-platform mapping (supports new platforms array field)
• URL state: ?os= param renamed to ?platform= with backward compat
• Hides memory/hardware panels when "Web" platform is selected
• Extracts filter types to model-filters.ts

Test plan

• Verify platform selector renders as a top-level grid section
• Switch between platforms and confirm draft state (selected models) persists per platform in localStorage
• Select "Web" platform and confirm memory/hardware panels are hidden and deploy tabs are hidden
• Verify ?platform=linux URL param works and ?os=linux still works for backward compat
• Check selected model cards show accent color backgrounds and borders
• Confirm model catalog correctly filters models by platform using resolveModelPlatforms()

[TimPietruskyRunPod]

Superseded by #156 which builds on this work — removed Web platform, added engine filter system (llama.cpp/MLX/wandler), and ONNX model support.