feat(webchat): add branding, avatars, and widget CSS hardening

[realcodesiman]

Summary

• Auto-contrast the widget's foreground text against the admin-chosen brandColor instead of a hardcoded white, and show the workspace logo beside bot/agent bubbles (gated server-side on showLogo).
• Sanitize IntegrationWebchat.customCss against </style> injection before it's rendered via dangerouslySetInnerHTML, and cap its length at 20k characters.
• Close a privilege gap: updateWebchatAction only checked workspace membership, so any member (not just superAdmin, as the edit page requires) could set customCss served to the public widget — now gated on superAdmin via a new workspaceMemberPermissions field on the action ctx.
• Small unrelated fixes bundled from the same working session: reload instead of toast+router.refresh() after workspace deletion actions, an account-rail plan label/layout tweak, and an add-channel-button anchor-instead-of-button fix.

Changes

• apps/builder/src/features/integration-webchat/lib/brand-color.ts (new) — WCAG relative luminance + readable foreground helpers
• apps/builder/src/features/integration-webchat/lib/widget-css.tsx (new) — sanitizeWidgetCss + CustomWidgetStyle server component
• apps/builder/src/features/integration-webchat/actions/update-webchat.action.ts, apps/builder/src/lib/safe-action.ts — superAdmin permission gate on the action, workspaceMemberPermissions exposed on ctx
• apps/builder/src/features/integration-webchat/schema/mutation.ts — customCss max length
• apps/builder/src/app/(no-sidebar)/webchat/page.tsx, webchat-wrapper.tsx, webchat-message-list.tsx, guest-session-provider.tsx, guest-sesssion-store.ts — wire brand color + workspace logo through to the guest widget
• apps/builder/src/features/messages/components/message-item.tsx — optional avatarUrl prop for agent/bot bubbles
• apps/builder/src/features/integration-webchat/dialogs/embed-code-dialog.tsx, components/authorized-domain-field.tsx — trim unused embed snippet options, add domain field placeholder
• apps/builder/src/features/workspaces/components/{account-rail,workspace-deletion-card}.tsx, apps/builder/src/features/inboxes/components/add-channel-button.tsx — unrelated small fixes
• Translation keys added across all apps/builder/messages/*.json locales
• New/updated tests under apps/builder/__tests__/ and apps/builder/src/features/workspaces/components/__tests__/

Test plan

• pnpm lint
• pnpm --filter builder check-types (required fixing an unrelated stale workspace link for @chatbotx.io/integration-instagram-facebook via CI=true pnpm install --no-frozen-lockfile)
• Manual verification: load a webchat widget with a light brandColor and confirm header/button text stays readable; toggle showLogo and confirm the avatar appears/disappears; attempt updateWebchatAction as a non-superAdmin member and confirm it's rejected