Upgrade msw-storybook-addon to v3 and msw to 2.15.0

[dgee2]

Bumps msw-storybook-addon 2.0.6 -> 3.0.0 and msw 2.12.14 -> 2.15.0
(these move together; msw-storybook-addon@2.0.6 is incompatible with
msw's newer worker internals, which is why Group 1 pinned msw down
via a pnpm override). Removes that override now that the addon
supports current msw.

msw-storybook-addon v3 removes initialize()/mswLoader in favor of
a definePreview-style addons: [addonMsw(setupFn)] call, and drops
parameters.msw support for CSF Next (this project's story format)
in favor of a beforeEach({ msw }) { msw.use(handler) } hook per
story. Updated .storybook/preview.ts and the three story files that
registered handlers accordingly.

This also surfaced and fixed two pre-existing, MSW-adjacent problems
that AGENTS.md had documented as "not currently reliable":

• MSW genuinely wasn't intercepting requests under
  vitest --project=storybook before (confirmed by probing — a
  stricter assertion against a previously "passing" story failed
  until the beforeEach migration). It works correctly now; updated
  AGENTS.md's component-testing guidance to match and removed the
  workaround comments in the affected story files.
• .storybook/msw-handlers.ts's recipe list fixtures used name
  instead of the real API's title field, which had gone unnoticed
  because MSW wasn't actually serving the response before.

Verified: lint, build, unit tests (57/57), storybook interaction
tests (78/78, now genuinely exercising mocked responses rather than
incidentally matching a fallback UI state), and a dev-server smoke
check. Playwright e2e (pnpm run test:e2e) fails, but on an
unrelated pre-existing stale scaffold assertion ("You did it!") that
doesn't match this app's actual content and is unmodified from main.

Co-authored-by: Claude noreply@anthropic.com

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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