chore(typescript): scope typechecking to src and add it to CI

[thetaPC]

Issue URL: internal

What is the current behavior?

tsconfig.json is three lines:

{
  "extends": "@docusaurus/tsconfig",
  "exclude": ["static/code/stackblitz/"]
}

Specifying exclude replaces TypeScript's defaults, and node_modules is one of them. So TypeScript walks node_modules and build/, pulling in 995 project files and 2,039 in total. Nothing runs tsc, so none of the 126 errors it reports have ever surfaced.

97 of those 126 are inside build/, which means the error count depends on whether you happen to have run a build. That scan is also load-bearing by accident: the real @theme/* types arrive only because node_modules is being walked, not because anything asks for them.

What is the new behavior?

npm run typecheck runs tsc --noEmit over 61 files at zero errors, and CI runs it.

Config:

• include scopes to src plus index.d.ts, the only declaration for *.module.scss. This also restores the default excludes, which is the actual fix for the above.
• types names @docusaurus/module-type-aliases and @docusaurus/theme-classic deliberately. Without it the catch-all declare module '@theme/*' in index.d.ts wins and every swizzled component becomes any.
• baseUrl is redeclared. Docusaurus sets it, but extends resolves it relative to the declaring file, so @site/* pointed inside node_modules/@docusaurus/tsconfig.
• allowArbitraryExtensions lets release-notes.d.json.ts type the generated release-notes.json, so typechecking needs neither a build nor a GitHub token.

Code:

• Declared ion-icon, docs-card and docs-cards in src/declarations.d.ts
• Added DocsFrontMatter for our demoUrl and demoSourceUrl front matter, intersected with Docusaurus's DocFrontMatter
• Removed a dead @stencil/core import left over from a Stencil to React port
• Declared @docusaurus/types, which @docusaurus/module-type-aliases imports from in 12 places and which resolved only because npm hoists it out of @docusaurus/core

Does this introduce a breaking change?

• Yes
• No

Other information

strict is not enabled. It surfaces 144 errors concentrated in four files, with Playground/index.tsx alone at 60. That belongs in its own PR, and it is worth doing before Docusaurus v4, which requires TypeScript 6, where strict defaults to on.

TypeScript stays at 5.9.3, the newest 5.x. 6 and 7 are blocked by @docusaurus/tsconfig setting baseUrl, which 6 deprecates and 7 removes outright. Docusaurus drops it in v4 (facebook/docusaurus#11915). The ignoreDeprecations: "6.0" escape hatch works, but it silences the whole deprecation class permanently, so it is not worth taking.

Ejected theme components are checked rather than skipped. The three errors in DocItem/Layout came from our own front matter fields, so the type lives beside it in frontMatter.interface.ts instead of being invented inline. The change to the upstream copy is 3 lines, all inside existing // CUSTOM CODE markers. The other 8 ejected files needed nothing once types was set.

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[brandyscarney]

LGTM