Podcasts with missing or broken artwork disappear from the grid #358

Problem. A saved podcast whose artwork URL is empty or fails to load has no card in the podcast grid, so the user cannot open it. Image.svelte renders only its fallback slot when the image errors (src/ui/common/Image.svelte:66-67), and PodcastGridCard.svelte passes no fallback (lines 15-21).

Observed. In Obsidian 1.13.7 on Linux, a saved feed with artworkUrl: "" rendered no .podcast-image card in the grid.

Acceptance. Every saved podcast shows a card with a readable placeholder (title initials or an icon) when its artwork is missing or broken, and the card opens the podcast. An end-to-end test covers a feed with broken artwork.

Found by the PodNotes stack evaluation, observed in Obsidian.