Typescript: Phase 1 — Tooling

[n-lark]

Phase 1 — Tooling

Zero changes to existing source files. Can land before Pinia completes.

• Add tsconfig.json to /frontend — extend Vue base, allowJs: true, strict: false
• Install typescript, vue-tsc as dev dependencies
• Update config/webpack.config.js — add ts-loader, extend resolve.extensions with .ts
• Add @typescript-eslint to ESLint, scoped to frontend/src/** (existing JS files unaffected)
• Mirror path aliases from frontend/jsconfig.json into tsconfig.json

After this PR merges: new files can be .ts immediately. Two rules apply from here:

1. All new frontend files are .ts (or <script lang="ts"> for Vue)
2. Convert existing files to TypeScript when touched for any other reason