Improve public project presentation and discovery

[gh615280-maker]

Summary

• replace the public landing README and add a complete Simplified Chinese version
• add real synthetic-data screenshots, social preview artwork, Apache-2.0 license, roadmap, issue forms, and a launch copy kit
• update h2 to 0.4.16 to address RUSTSEC-2026-0258
• make fixture hashes deterministic across Windows checkouts
• use an in-memory SQLite reference during restore schema preflight to avoid transient Windows file locks

Validation

• npm.cmd run format:check
• npm.cmd run check:sensitive
• npm.cmd run check:licenses
• npm.cmd run fixtures:verify
• git diff --check
• cargo metadata --manifest-path src-tauri/Cargo.toml --locked --no-deps --format-version 1
• cargo tree --manifest-path src-tauri/Cargo.toml --locked -i h2
• focused restore test: restore_is_preflighted_then_atomically_installed_only_during_restart_recovery

Notes

• screenshots contain synthetic test data only
• repository topics, description, homepage, and Discussions were updated separately through GitHub settings
• the local TextbookLens_User_Guide.docx remains untracked and is not part of this PR