Introduce domain-first src/ui and src/app layout; update build config and scripts

[stefan-burke]

Motivation

• Introduce a domain-first layout to separate runtime entrypoints (src/app) and UI assets (src/ui/*) to support an incremental migration and clearer boundaries.
• Update tooling, build scripts and import maps to reference the new locations for client bundles, static assets and templates.

Description

• Move UI code from src/client, src/static, and src/templates into a new src/ui subtree and update references across the codebase.
• Add new runtime bootstraps src/app/index.ts and src/app/edge.ts and make src/index.ts and src/edge.ts lightweight backwards-compatible import wrappers.
• Update build and helper scripts to use the new paths (scripts/build-static-assets.ts, scripts/build-edge.ts, scripts/find-unused-src.ts) and inline static asset reads from src/ui/static.
• Update project configuration and tooling: deno.json, biome.json, and tsconfig.json import paths, lint/format excludes, and override entries; add REPO_STRUCTURE.md and link it from README.md.

Testing

• No automated tests were executed as part of this change; CI is expected to run the standard test and lint suites (for example deno task test and deno task lint).

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Codex Task