Rename frontend to app, embed build outputs, and update CI/Taskfile

[fdatoo]

Motivation

• Standardize the frontend crate name from web to app across build tooling and CI so local layout and CI targets match the repo structure.
• Ensure the SPA build output is treated as a committed artifact for embedding and CI (keep a .gitkeep placeholder and avoid accidental deletion).
• Harden index rendering and asset handling to work with minimal builds and adjust tests to the new app markup.

Description

• CI: replaced web references with app in .github/workflows/ci.yml, renaming job outputs/filters and updating Node cache paths and steps to run npm ci, npm run typecheck, npm test, and npm run build against app.
• Taskfile: renamed web:* tasks to app:* (install/build/typecheck/test/dev) and updated build task dependency to use app:build.
• Frontend build: added app/scripts/clean-web-dist.mjs and updated app/package.json build script to run the cleaner before vue-tsc + vite build.
• Vite: set build.outDir to ../internal/web/dist, emptyOutDir: false, and added rollup manual chunking for monaco in app/vite.config.ts.
• Embedding & assets: added internal/web/dist/.gitkeep and updated .gitignore to keep the placeholder while ignoring other dist files.
• Server and templates: changed the index root element from id="root" to id="app", made renderIndex tolerant to missing dist by allowing fs.ErrNotExist to be treated as non-fatal, and updated NewHandler docs and initialization to use the embedded app bundle.
• Tests: updated internal/web tests to assert on id="app" and refined the asset budget test skip logic and existence checks.

Testing

• Ran task build and task app:build which invoked the Node build and Go build workflows and completed successfully.
• Ran unit tests with go test ./... including updated internal/web tests and they passed.
• Ran frontend checks npm run typecheck and npm test in app and they completed successfully.

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Codex Task