Modernize frontend/build stack for Svelte 5

[rendall]

Summary

This PR completes the Priority 4 frontend/build modernization lane.

It upgrades the frontend stack to the current Svelte 5-compatible toolchain and carries through the compatibility work needed to keep build, tests, local runtime, and embed mounting working.

Key changes:

• upgrade the frontend stack to svelte@^5.55.0, @sveltejs/vite-plugin-svelte@^4.0.0, and svelte-preprocess@^6.0.3
• align the Vite/Svelte preprocess path to tsconfig.frontend.json
• align frontend Jest handling for the post-upgrade ESM dependency edge (esm-env)
• enable verbatimModuleSyntax and move frontend module resolution to bundler
• fix the Svelte 5 invalid non-void self-closing tags surfaced by the build
• convert StateValue imports to type-only imports to remove build noise
• migrate src/simple-comment.ts from new SimpleComment(...) to mount(SimpleComment, ...)
• archive the completed Priority 4 dependency/frontend documentation artifacts

Validation

Completed on the frontend-modernization branch during the upgrade loop:

• yarn lint
• yarn run prettier --list-different .
• yarn typecheck
• yarn test:frontend
• yarn run build:frontend
• yarn run ci:local

Local runtime verification:

• simple-comment mounts again in local development after the Svelte 5 mount(...) change
• once MongoDB/backend runtime was started, the app ran successfully end-to-end

Notes

A later ci:local rerun after local Netlify dev/runtime activity picked up generated .netlify/functions-serve/* artifacts during linting. That is documented in the archived validation notes as generated-artifact/environment noise rather than a frontend/build regression from this PR.

[Copilot]

Pull request overview

Modernizes the repo’s frontend/build toolchain to a Svelte 5–compatible stack while preserving existing embed/runtime behavior and keeping lint/test/build workflows working.

Changes:

• Upgrades Svelte/Vite/Svelte-preprocess/ts-jest dependencies and lockfile to the Svelte 5 ecosystem.
• Aligns frontend build + test configs to the updated TS/ESM model (tsconfig.frontend.json, Vite preprocess, Jest ESM edge cases).
• Applies required source compatibility fixes (Svelte 5 mount(...), type-only imports, and non-void self-closing tag fixes).

Reviewed changes

Copilot reviewed 17 out of 18 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
yarn.lock	Updates resolved dependency graph for the upgraded Svelte 5-compatible toolchain.
package.json	Bumps core frontend/build dependencies (Svelte 5, Vite plugin, preprocess, ts-jest, etc.).
vite.config.ts	Pins Svelte preprocess TypeScript settings to tsconfig.frontend.json.
tsconfig.frontend.json	Enables verbatimModuleSyntax and switches module resolution to bundler.
jest.frontend.config.ts	Adjusts Jest transforms for ESM deps (esm-env) and treats .ts as ESM.
eslint.config.mjs	Tweaks unused-vars ignore pattern in Svelte files to avoid noise around state.
src/simple-comment.ts	Migrates embed entrypoint from new Component(...) to mount(Component, ...).
src/simple-comment-icebreakers.ts	Adds missing TypeScript types for async callback parameters.
src/components/low-level/SkeletonText.svelte	Replaces invalid non-void self-closing tags with explicit closing tags.
src/components/Login.svelte	Converts StateValue import to type-only.
src/components/DiscussionDisplay.svelte	Fixes invalid self-closing non-void <div />.
src/components/CommentInput.svelte	Converts StateValue import to type-only; fixes invalid self-closing <textarea />.
src/components/CommentEdit.svelte	Converts StateValue import to type-only; fixes invalid self-closing <textarea />.
src/components/CommentDisplay.svelte	Fixes invalid self-closing non-void <div /> placeholders.
docs/archive/Priority4FrontendBuildModernizationPlan.md	Archives the Priority 4 frontend/build modernization plan artifact.
docs/archive/Priority4FrontendBuildModernizationChecklist01Validation.md	Archives validation evidence/notes for the lane completion.
docs/archive/Priority4FrontendBuildModernizationChecklist01.md	Archives the checklist artifact for the completed lane.
docs/archive/Priority4DependencyModernizationPlan.md	Marks the broader Priority 4 dependency modernization plan as archived.

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[Copilot]

svelte-preprocess v6 still supports this call shape, but its default export is deprecated. To avoid deprecation warnings and reduce risk of a future breaking change, switch to the named export (import { sveltePreprocess } from "svelte-preprocess") and call that here.