chore(studio): remove unused dependencies found by knip (#49720)

Removes the Studio dependencies knip reports as unused, and declares one
it reports as unlisted. Second PR in the stack (on top of #49719,
followed by #49721 which adds the CI gate).

**Removed:**
- `@ai-sdk/provider`, `@ai-sdk/provider-utils` — zero references
- `eslint-plugin-jsx-a11y` — the `jsx-a11y/*` rules resolve through the
plugin registered by `eslint-config-next` (via
`eslint-config-supabase/next`); verified 259 a11y warnings still fire
after removal
- `common`, `config` from `devDependencies` — duplicates of the
`dependencies` entries

**Added:**
- `@tailwindcss/postcss` as a Studio devDependency —
`apps/studio/postcss.config.cjs` loads it (through
`config/postcss.config`), but only `packages/config` declared it, so
under pnpm's strict isolation it was never resolvable from Studio's own
`node_modules`

**Kept deliberately** (nothing imports them by a specifier knip can
follow, but removing them breaks things — they get `ignoreDependencies`
entries in #49721): `lodash-es` (string-resolved in `vite.config.ts`),
`raw-loader` (loader string in `next.config.ts`), `import-in-the-middle`
/ `require-in-the-middle` (Sentry/OTel runtime hooks, #35030),
`@babel/core` (resolution pin, #45876).

Heads-up on the lockfile: ~500 of the lines are pnpm re-resolving
`apps/www`'s stale auto-installed vitest peer from `vite@6.4.3` →
`8.2.1` (www doesn't depend on vite directly; Studio already runs vitest
on vite 8). Any dependency change triggers it — not specific to this PR.

## To test

- `pnpm install --frozen-lockfile` succeeds
- `pnpm dev:studio` — Tailwind styles still apply (the postcss plugin
now resolves from Studio)
- `pnpm lint --filter=studio` still reports `jsx-a11y/*` warnings, no
"Definition for rule not found"
- `pnpm --filter www test` (www's vitest now runs on vite 8)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
  * Updated Studio’s development tooling configuration.
  * Removed unused package dependencies and development tools.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Alaister Young <10985857+alaister@users.noreply.github.com>

apps/studio/package.json

@@ -42,8 +42,6 @@
     "@ai-sdk/amazon-bedrock": "5.0.55",
     "@ai-sdk/mcp": "2.0.31",
     "@ai-sdk/openai": "4.0.41",
-    "@ai-sdk/provider": "4.0.7",
-    "@ai-sdk/provider-utils": "5.0.26",
     "@ai-sdk/react": "4.0.62",
     "@aws-sdk/credential-providers": "^3.1041.0",
     "@configcat/sdk": "^1.1.0",
@@ -171,6 +169,7 @@
     "@graphql-codegen/cli": "5.0.5",
     "@graphql-typed-document-node/core": "^3.2.0",
     "@jridgewell/remapping": "^2.3.5",
+    "@tailwindcss/postcss": "^4.2.4",
     "@tailwindcss/vite": "4.2.4",
     "@tanstack/devtools-vite": "^0.6.0",
     "@testing-library/dom": "^10.0.0",
@@ -198,12 +197,9 @@
     "@vitest/ui": "catalog:",
     "api-types": "workspace:*",
     "autoevals": "^0.0.132",
-    "common": "workspace:*",
-    "config": "workspace:*",
     "date-fns": "^2.30.0",
     "eslint-config-supabase": "workspace:*",
     "eslint-plugin-barrel-files": "^2.0.7",
-    "eslint-plugin-jsx-a11y": "^6.10.2",
     "eslint-plugin-react-hook-form": "^0.3.1",
     "eslint-plugin-valtio": "^0.8.0",
     "graphql-ws": "5.14.1",