refactor: unify deploy base path handling

[joyenjoyer]

This change is needed to be able to host PR previews on Github pages of instructure-design-tokens repo.

Unify docs-app deploy base path handling

Make PUBLIC_PATH (env var → webpack output.publicPath) the single source of truth for "where this build lives" and thread it through every layer:

• webpack.config.mjs — resolve PUBLIC_PATH once and pass it to both output.publicPath and HtmlWebpackPlugin's templateParameters.
• index.html — <base href> and the inline 404 SPA-redirect script both read the template parameter; the redirect is now generic instead of pattern-matching pr-preview/latest.
• navigationUtils.ts — new getDeployBase() reads __webpack_public_path__ at runtime. parseCurrentUrl strips it before parsing the in-app route; getAssetBasePath returns it directly. Renamed
  prPrefix → basePrefix and de-duplicated a double parseCurrentUrl() call in buildUrl.

No behavior change for current instructure.design deploys. Unlocks hosting the docs-app at any base path — other repos (e.g. design-tokens previews) or arbitrary sub-hosts — by setting PUBLIC_PATH at
build time.

[github-actions]

PR Preview Action v1.8.1
🚀 View preview at https://instructure.design/pr-preview/pr-2557/
Built to branch gh-pages at 2026-05-12 14:13 UTC. Preview will be ready when the GitHub Pages deployment is complete.

[github-actions]

Visual regression report

✅ No changes.

Status	Count
Unchanged	32
Changed	0
New	0
Removed	0

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