fix(settings): restore paste-to-destructure for workspace secrets, cleanup hooks and design tokens

Restores the env-var paste feature (KEY=VALUE → split rows, multi-line
→ multi rows) for workspace secrets that was lost when the unified
Credentials tab was split into Secrets and Integrations. Adds
`parseEnvVarLine`, `parseValidEnvVars`, and `handleWorkspacePaste` with
full support for export prefix, quoted values, inline comments, and
base64 false-positive guards. Also adds consistent value masking
(show on focus / mask on blur) to new workspace input rows.

Cleans up ~20 unnecessary `useCallback` wrappers, fixes a direct state
mutation in `handleSingleValuePaste`, moves `e.preventDefault()` inside
the `parsedVars.length > 0` guard, replaces all hardcoded hex colors
with CSS variable tokens, converts template-literal classNames to `cn()`,
and replaces raw `<button>` with emcn `Button`.

Author: waleedlatif1