[dimslaev]

Chunk 1 of 6 of the frontend FSD + kebab-case move. Nothing moves yet. This PR adds the tests and linters the later chunks rely on.

Changes

• Specs for code the later chunks touch that nothing covered:
  • feed-filters.spec.ts: each filter row (From, Topics, Published, Sort by, search) narrows the feed, and the list matches the response.
  • rails.spec.ts: on wide screens the header spans the inline rails on the feed and narrows on /profile. On mid screens the rail button shows on the feed only, and [ opens and closes the sheet but not while typing in search.
  • admin.spec.ts: an admin opens analytics from the user menu and changes the window. A reader sees "Admins only." Logged out, /admin redirects to login.
• Steiger 0.7 with the FSD plugin, run by bun run lint:layers, plus a CI step in test-frontend.yml with continue-on-error (report only). fsd/insignificant-slice is off: Steiger only counts references from FSD layers, and routes/ isn't one, so every slice imported only by a route shows as unused.
• Biome useFilenamingConvention (kebab-case) at warn. It passes __root.tsx, _layout.tsx, $id.tsx and foo.index.tsx without extra config.
• ADR 3 and scripts/check-banned-module-names.sh now allow frontend/src/shared/, the FSD layer name Steiger requires. The hook still checks every name below it.
• src/index.css: bun run lint reformats one rule that was committed unformatted on master.

Findings from the spike

• Steiger accepts routes/ above the layers. It catches feature→feature and upward imports. It does not catch routes/ importing past a slice's index.ts.
• Steiger rejects files placed directly in a slice (no-segmentless-slices), so slices use ui/, model/ and lib/ segments.
• With aliases set to @/shared/*, the shadcn CLI writes into src/shared/ui/ and rewrites imports correctly. I tested against a stub registry because ui.shadcn.com is blocked from this sandbox.
• Two imports in the plan's mapping would be forbidden. Both are settled: LikeButton goes into features/feed, and AuthLayout becomes a pathless routes/_auth.tsx.

Checks

• bun run build green.
• Playwright: 44 passed on master, 55 passed on this branch (local stack: seeded db, backend, Vite).
• pre-commit hooks pass. zizmor needs a GitHub token to run its online audits here; run offline on test-frontend.yml it reports no findings.

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