feat: encrypted-file vault, sandbox-safe IO, share links, folder upload

[huhn511]

Summary

A cohesive batch of Drive improvements centered on encrypted files and the Mind-shell iframe, plus a couple of UX refinements.

• Passphrase vault — a per-user, in-pod encrypted index mapping each encrypted file to the passphrase that opens it, searchable behind a new Vault dialog. Stored as .vault.enc + sidecar (AES-GCM under the session passphrase), hidden from the listing. Honors the privacy invariants: never leaves the pod in plaintext, no central DB.
• Sandbox-safe download & clipboard — new saveBlob() / copyText() helpers. Inside the shell's sandboxed iframe, <a download> and the async Clipboard API are silently blocked; these fall back (open-in-new-tab / execCommand + manual-select) so download and "copy link/passphrase" work embedded too.
• Share encrypted files — grants/revocations apply to the ciphertext and its sidecar together, and the shareable link is the in-app viewer link (?src=) that prompts the recipient for the passphrase. FilePreview can now render any pod URL read-only and anonymously (owner-only actions hidden).
• Extension-locked rename — renames edit the base name only; the extension is locked so a rename can't change the file format. Encrypted renames move the .enc.json sidecar alongside and update originalName.
• Upload into a folder — drop files onto a folder row/tile to upload into it (page-level dropzone still handles the current folder).
• Polish — breadcrumb icon + heading redesign; image thumbnails in list view.

Review notes

• typecheck: clean for all touched files. (The 3 pre-existing layout.tsx / opengraph-image.tsx errors are a stale @mind-studio/ui in node_modules, present on main and unrelated.)
• biome check: clean on all changed files.
• Known follow-ups (non-blocking): removeVaultEntry exists but isn't wired into delete yet, so the vault keeps a stale entry after an encrypted file is deleted.

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