add React docs section (@cofhe/react) and replace quick-start stub

[roeezolantz]

Summary

@cofhe/react is a fully-shipped library (currently 0.5.2) with providers, hooks for connection / permits / encryption / decryption / contract reads & writes / token helpers, and a styled UI kit including the CofheFloatingButton portal. The docs had only one page about it — a stub quick-start saying it was "currently in development." This PR replaces the stub and adds a real three-page React section.

Together this covers gaps B-1 and A-5 from the docs audit on XDL-11.

Where the underlying changes were made

All shipped in @cofhe/react@0.5.0, with two patches since:

• useCofheEnabled hook — changeset `09bf7c9`. Reads TaskManager.isEnabled() reactively.
• onPoll callback end-to-end — changeset `788a6e2`. See also docs PR #20 for the corresponding SDK guide updates.
• Permit validation tightening + disabledDueToMissingPermit → disabledDueToMissingValidPermit rename — changeset `9a06012`. Source: `useCofheReadContract.ts#L183-L231`, `useCofheReadContractAndDecrypt.ts#L77-L100`, `useCofheTokenDecryptedBalance.ts#L34-L84`.
• Custom-token import in the portal + persisted via useCofheTokens — changeset `7b1f4c3`.
• Wallet-disconnected portal state + react.projectName — changeset `f857263`.
• Configurable internal logger via createCofheConfig({ react: { logger } }) — changeset `503536a`.
• MUI icon peer dep removed (icons bundled) — changeset `90a0d02`.
• SSR safety (@cofhe/sdk/web lazy-loads tfhe) — `@cofhe/sdk@0.5.1` — `342fd0f`.

Source layout (every reference is from packages/react/src/):

• Provider: `providers/CofheProvider.tsx`.
• Hook exports: `hooks/index.ts`.
• README the docs draw from: `packages/react/README.md`.

Changes

File	What changed
client-sdk/react/getting-started.mdx	New. Install + peer deps (with the MUI removal note), CofheProvider props table (config-only vs pass-an-existing-client modes), the react.* config options table (projectName, logger, position, initialTheme), hooks-only vs @cofhe/react/ui styled-UI entrypoints, SSR notes.
client-sdk/react/hooks.mdx	New. Hook reference grouped into: Client and connection, Coprocessor status (useCofheEnabled), Permits (typed ValidationResult.error), Encryption (useCofheEncrypt, useEncryptInput), Reading encrypted state (with the disabledDueToMissingValidPermit rename <Note>), Writing encrypted state, and the full CoFHE token-helper surface. Cross-references the TaskManager and Permits deep-dives.
client-sdk/react/components-and-portal.mdx	New. CofheEncryptInput props table, the CofheFloatingButton portal mount snippet, wallet-disconnected projectName behavior, position/theme config, the custom-token import flow, bundling notes (CSS auto-load on /ui, MUI peer removed).
client-sdk/quick-start/react.mdx	Replaced the stub. Real install + provider + decrypted-balance walkthrough, with links into the new React section.
docs.json	Added the "React" group under the Client SDK tab, between "Foundry Plugin" and "Examples", with the three new page IDs.

Test plan

• `mint dev` shows a new "React" group in the Client SDK sidebar with three reachable pages.
• The replaced React quick-start no longer says "in development" and instead links into /client-sdk/react/*.
• Cross-references to existing pages resolve:
  • /client-sdk/guides/permits#validating-permits
  • /deep-dive/cofhe-components/task-manager#enable--disable-switch
  • /client-sdk/react/getting-started#react-config-options