Site-wide performance tune-up

⚡ Performance Country list slimming – the country dropdown no longer pulls in the full world-atlas JSON. A pre-computed name list ships instead, removing about 100KB from every page's initial load. Parallel profile loading – when you're signed in, your profile and favorite countries now load in parallel instead of one after the other, shaving a database round-trip off every page navigation. Lazy post thumbnails – images in post lists now defer loading until they scroll into view and reserve their space in advance. Scrolling feels smoother (no more subtle layout jumps) and off-screen images don't eat your data anymore.