fix: support .htm files in addition to .html

[VedantMadane]

Fixes #10997. Vite now properly handles .htm files. Updated htmlFallback, static, indexHtml, and memoryFiles middlewares to use the existing htmlLangRE regex that matches both .html and .htm extensions.

[VedantMadane]

@sapphi-red when should this PR be marked ready for review?

[sapphi-red]

When the CI passes

[VedantMadane]

Reset this branch onto latest main with a single focused commit (the previous update accidentally replayed unrelated upstream history). Diff should now only touch Vite HTML handling for .htm support per #10997.

[VedantMadane]

@sapphi-red Branch was reset to a single commit on current main (the earlier tip had accidentally replayed unrelated history). Pushed an oxfmt-style formatting fix for the lint failure; once CI is fully green this should be ready for review per your note. Thanks for the pointer.

[VedantMadane]

Rebased onto latest main to re-run CI.

Prior CI failures on Build&Test node-24 macos/windows looked flaky/unrelated to the .htm changes:

• playground/import-assertion hook timeout (~50s)
• transient "server is being restarted or closed" errors
• html.spec.ts / indexHtml.spec.ts had already passed

Locally after rebase: unit tests for packages/vite/src/node/__tests__/html.spec.ts and packages/vite/src/node/server/middlewares/__tests__/indexHtml.spec.ts all passed (15/15). No merge conflicts during rebase; .htm handling preserved across client/build/plugins/middleware sites.