perf(frontend/mfa): lazy-load qrcode out of main bundle (closes #511)

[cristim]

Summary

• auth.ts already uses import('qrcode') dynamically (PR feat(auth+frontend/mfa): finish MFA enrollment + login two-step (closes #497) #509), but the vendor splitChunks cache group had chunks: 'all', which captured async imports too -- so qrcode still ended up in vendors.js on every page load.
• Added a dedicated qrcode cache group (chunks: 'async', priority 20) so webpack emits it as its own on-demand chunk.
• Changed the vendor cache group to chunks: 'initial' (priority 10) so only synchronously-imported packages land in vendors.js.

Bundle output (production build)

Before:

js/vendors.175b9550.js  224 KiB   [initial, in main entrypoint]
Entrypoint app: 608 KiB

After:

js/vendors.4e1021cc.js        200 KiB   [initial, in main entrypoint]
js/qrcode.f45fe2e9.chunk.js    22.6 KiB  [async, loaded on first enrollment click]
Entrypoint app: 584 KiB  (-24 KiB)

vendors.js shrank by 24 KiB (10.7%). The qrcode chunk is fetched only when the user clicks "Set up two-factor authentication" in the profile modal.

Test plan

• npx tsc --noEmit -- no type errors
• npx jest -- 2142 tests pass, 0 failures (including auth-mfa-enroll.test.ts)
• npm run build -- qrcode.*.chunk.js present in dist/js/, absent from entrypoint list
• Manual: open profile, observe no qrcode chunk in Network panel until "Set up two-factor authentication" is clicked

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ℹ️ Review info

⚙️ Run configuration

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Run ID: 2480d2b7-3052-4d75-88f4-4edf8a3723f3

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Reviewing files that changed from the base of the PR and between 4956d66 and 313ed21.

📒 Files selected for processing (1)

• frontend/webpack.config.js

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• Commit unit tests in branch fix/511-wave4

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