Add Confidential mode docs page

[charitra-prem]

What

Adds a docs page for Prem confidential compute (PCCI) inference, plus nav and spell-check updates.

• New page features/confidential.mdx ("Confidential mode"), added to the Features sidebar group.
• Six screenshots under images/features/confidential/, captured from the app driven against the dev backend.
• hypervisor allowlisted in .cspell.json; crypto identifiers are code-formatted so cspell ignores them.

Page contents

• What is confidential compute — open-source models in Prem's Enclave, hosted in a TEE; every request encrypted with a key only the user holds; no one (Prem, cloud provider, physical access) can read it.
• How your data is secured — encrypted in transit, at rest, and in use (the model runs inside the TEE).
• How the protection is enforced — hardware-signed attestation, "don't trust the operator, trust the signature", post-quantum XWing (X25519 + ML-KEM 768). Adapted from docs.prem.io.
• Plans — confidential mode is Max-only; accounts start on a sponsored Pro plan.
• Turning it on — 3-step flow with screenshots: switch to Max, enable, confirm active (green widget).
• FAQ — permanence (one-way switch), behaviour when inference is unavailable (no silent fallback), what's disabled in the mode, plan needed, privacy guarantee.

Validation

• cspell passes.
• docs.json is valid; the page is in nav; all six referenced images resolve.
• Rendered locally with mint dev (Steps, Frames, Warning, and FAQ accordions all render; prev/next is Memory ↔ Telegram).

Note: the existing resources/pricing.mdx page still describes confidential computing on Pro, whereas the live app gates it to Max. Worth reconciling in a follow-up.

🤖 Generated with Claude Code