Implement platform-specific modules approach like Saikuro

okay so the ui/penumbra-ui/src/lib.rs had gotten GROSS. it was this soup of #[cfg(not(target_family = "wasm"))] and #[cfg(target_family = "wasm")] blocks sitting right next to each other, two versions of half the functions, the SharedState struct declared twice, the boot path forked inline. every time I added a thing I had to add it twice with the right cfg gate and it was exactly the kind of mess I swore off on day one.

so I stole the pattern from Saikuro

a platform module. now there’s src/platform/mod.rs that declares the shared interface, plus platform/native/mod.rs and platform/wasm/mod.rs behind one cfg switch at the top. the rest of the app just calls into platform:: and doesn’t care which one it got. all the two-versions-of-everything nonsense is now contained in exactly two files that never both compile at once, and lib.rs reads like a single coherent app again instead of a choose-your-own-adventure. it’s so much cleaner.

while I was in the editor I also killed a dumb duplication: penumbra-editor had its OWN BlockKind enum that was just a lossy copy of the one in penumbra-markdown, with a whole block_kind() function to convert between them. why. they’re the same concept. so now the editor re-exports markdown’s BlockKind and clones it straight through, and that whole translation layer is gone.