[codex] Harden Tauri app baseline

[saagpatel]

What

Hardens the DevToolsTranslator local-first Tauri baseline by adding the missing Tauri dev URL/dev command, default capability, generated capability schema, preview script plumbing, Codex verify runner, and reusable demo/evidence documentation surfaces.

Why

The cross-app local-first Tauri scanner flagged baseline drift: incomplete Tauri build config, missing default capability, missing root preview script, missing Codex verify runner, and missing demo/evidence artifact paths. This brings DevToolsTranslator into the shared internal app template/checklist pattern.

How

• Added beforeDevCommand and devUrl to tauri.conf.json for the desktop UI dev server.
• Added a scoped default Tauri capability for the main window and refreshed the generated capability schema.
• Added root and desktop UI preview scripts that serve .vite-dist on 127.0.0.1:1420.
• Added .codex/scripts/run_verify_commands.sh to execute .codex/verify.commands.
• Added README skeletons for demo plan, screenshot capture plan, one-pager outline, and deck outline using sanitized/local-first evidence rules.

Checklist

• Tests pass
• No new warnings
• Documentation updated (if applicable)

Validation run locally:

• local-first Tauri scanner with --fail-on warning: 0 findings
• Prettier check on touched docs/package/config files
• JSON parse checks
• bash -n .codex/scripts/run_verify_commands.sh
• git diff --check
• pnpm --filter @dtt/desktop-ui build
• cargo fmt --all -- --check
• cargo test -p dtt-desktop-core
• cargo check -p dtt-desktop-core --features desktop_shell