refactor(dashboard): migrate env-var vault calls to buf-generated connect TS client

[coderabbitai]

Summary

Currently, the dashboard manually defines TypeScript types and fetch wrappers for vault RPC calls (e.g., EncryptBulkRequest, EncryptBulkResponse, vault.encryptBulk) in web/apps/dashboard/lib/vault.ts.

The proto definitions already exist; we should use buf to generate a TypeScript connect client instead of maintaining hand-written fetch wrappers and type definitions.

Action Items

• Use buf to generate a TypeScript connect client from the existing vault proto definitions
• Migrate all env-var related vault calls (encrypt, encryptBulk, decrypt, etc.) to use the generated connect client
• Remove the hand-crafted types and fetch wrappers from web/apps/dashboard/lib/vault.ts

Context

• PR: fix(dashboard): call vault encryptBulk for sequential bulk insert #6035
• Discussion: fix(dashboard): call vault encryptBulk for sequential bulk insert #6035 (comment)
• Requested by: @chronark

[linear-code]

ENG-2786