Corrected social metadata for public spec pages

OpenGraph images for specs now use paths ending in .jpg, which helps social platforms and caches recognize the image format correctly. The server still accepts these URLs and validates the hash the same way as before. Spec page titles also update through LiveView navigation, so the browser tab and social metadata reflect the current spec when moving between pages. A related visual fix normalizes the feed dropdown icon button size so it lines up with the other spec page controls.