Issue #6: Netflix-style cover tile screens for Movies and Series

Forgejo: http://100.120.38.37:3001/from-admin/riptide/issues/6

Summary

After entering the parent Movies or Series options in the RipTide library, the screen should present a tile-first browsing experience with cover art, closer to a Netflix-style media shelf/grid, instead of feeling like a plain title list.

Desired Behavior

• Clicking Movies opens a cover-art tile screen by default.
• Clicking Series opens a cover-art tile screen by default.
• Tiles use poster/cover art with placeholders while async poster fetch/decode completes.
• Navigation remains responsive with existing poster fetch/decode safeguards.
• Selected tile has a clear focus treatment and enough metadata to identify the title.