chore: use .ts extensions in TypeScript imports

[bartveneman]

Moves the .js → .ts import extension changes out of #402 into their own PR, so #402 can be rebased on top and only show the theme toggle changes.

• 101 files, import specifier changes only ('#lib/foo.js' → '#lib/foo.ts'); same set of files as feat: add theme toggle button to CmdK dialog #402
• No behavior changes. The .svx files still use .js imports, as they do in feat: add theme toggle button to CmdK dialog #402

Checks: pnpm check (0 errors) and pnpm test:unit (114 passed). lint:ts (mdsvex.config.js) and format:check (src/app.html, group-by-year.test.ts) also fail on main; this PR doesn't cause those failures.

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https://claude.ai/code/session_01C78rhsv2CRgCow5a5SDq6o

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[bartveneman]

Audit is failing, but not because of this PR. pnpm audit reports 10 vulnerabilities (6 high, 4 moderate) in dependencies. One example is js-yaml <4.3.2, pulled in via stylelint > cosmiconfig. This PR doesn't change package.json or pnpm-lock.yaml, and the same check also fails on #402. None of the open PRs fixes it yet, so it needs its own dependency update. I'm not adding one here so this PR stays limited to the import changes.

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