Standardize wasm examples: unify seven_guis/todomvc, add HOWTO_WASM.md

#429 in DioxusLabs/blitz — merged 2026-05-11

Renames wasm_7guis → seven_guis and adds wasm support to todomvc, giving both examples a unified dual native/wasm structure under the workspace. Consolidates build instructions into HOWTO_WASM.md, adds justfile targets for wasm builds, and updates the CI build-wasm-examples job to match.

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