Portal (#6391)

# Description of Changes

## What & why

This PR introduces the **Stirling developer portal** — a new
control-plane frontend that sits alongside the existing PDF editor —
plus the shared design system and workspace structure needed to host
both apps in one frontend.

The portal is the parent product surface: where users connect sources,
compose pipelines, wire agents, and manage usage / billing /
infrastructure, with the PDF editor as one capability inside it. This PR
lays the **foundation** — workspace reshape, design system, app shell,
navigation, and a mock-driven home — rather than wiring real backends
(those surfaces are placeholders for follow-up phases).

## What's in this PR

**1. Frontend repo reshape (`frontend/src/` → `frontend/editor/`)**
The existing editor app moved under `frontend/editor/`, so `editor`,
`portal`, and `shared` are siblings in one workspace. All references
were updated accordingly: `LICENSE`, `.dockerignore`, `.gitignore`,
build/sign shell scripts, the GH language-check script, the Taskfile,
and Docker config. **No editor source logic changed — path references
only.**

**2. New shared design system (`frontend/shared/`)**
- **Design tokens** in `tokens.css` as the single runtime source of
truth (light/dark, category accents, gradients). `tokens.ts` now holds
only the `Tier` type — the old JS palette mirror was removed (nothing
consumed it and it had drifted).
- ~30 framework-light **components** (Card, Button, Input, Select, Tabs,
Modal, Drawer, Toast, MetricCard, StatusBadge, Skeleton, EmptyState, …)
with Storybook stories.
- **Typed data catalogues**: `endpoints.ts` (10 verticals / 64
endpoints) and `ops.ts`.

**3. New developer portal app (`frontend/portal/`)**
- App shell: `Header`, `Sidebar`, `AssistantPanel`, search modal,
notifications, tier switcher, theme toggle, MSW toggle.
- **Tier-aware** home (free / pay-as-you-go / enterprise): KPI strip,
30-day usage chart, onboarding checklist, quick actions, recent
activity, region health, product grid, and a curated **"Popular use
cases"** teaser.
- **Documents** view hosting the full, tab-filterable endpoint
catalogue.
- Placeholder views for Sources / Pipelines / Agents / Editor /
Infrastructure / Usage & Billing / Developer Docs / Settings (follow-up
phases).
- **MSW-mocked** API layer: `api/*` issues real `fetch`, intercepted by
mocks in dev/Storybook; pointing at a real backend is just a matter of
not registering MSW. `react-router` URLs; Tier / View / UI contexts.

**4. Tooling & guardrails**
- ESLint extended to `portal` + `shared`, with **layering-boundary
rules**: `shared/` may depend only on third-party packages and itself
(no `@app` / `@portal` / `@core` / `@proprietary` / Tauri), so it stays
cleanly extractable into a standalone package later.
- `dpdm` circular-dependency check now walks editor + portal + shared
(the old glob matched only 2 files).
- New **devDependencies only** — Storybook (+ a11y/docs/themes addons),
MSW. No runtime dependencies added.
- New tasks: `frontend:dev:portal`, `frontend:build:portal`.

## Testing done locally

- `tsc` for both `portal` and `shared` projects — clean
- `eslint --max-warnings=0` across the whole frontend — clean
- `dpdm` circular-dependency check — no cycles
- Editor builds clean: `vite build editor --mode core` (✓ built, only
the pre-existing >500 kB chunk-size advisory)
- Editor runs in dev (core mode) with **zero console errors**; portal
runs in dev across all three tiers

## Notes for reviewers

- The change is overwhelmingly **additive**: `shared/` and `portal/` are
brand-new; the existing editor is path-reference changes only.
- The portal is intentionally **mock-driven** at this stage — real
backends and the remaining views land in follow-up phases.

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have run `task check` to verify linters, typechecks, and tests
pass
- [x] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

---------

Co-authored-by: Claude Opus 4.8 (1M context) <[email protected]>

frontend/shared/components/PanelHeader.tsx

@@ -0,0 +1,60 @@
+import type { ReactNode } from "react";
+import "@shared/components/PanelHeader.css";
+
+export interface PanelHeaderProps {
+  title: ReactNode;
+  /** Sub-heading below the title. */
+  subtitle?: ReactNode;
+  /** Show a back chevron and trigger this callback when clicked. */
+  onBack?: () => void;
+  /** Right-aligned action buttons / chips. */
+  actions?: ReactNode;
+  className?: string;
+}
+
+/**
+ * Header strip used by drill-down panels (admin tabs, agent detail, settings
+ * sub-pages). Back chevron renders only when `onBack` is supplied.
+ */
+export function PanelHeader({
+  title,
+  subtitle,
+  onBack,
+  actions,
+  className,
+}: PanelHeaderProps) {
+  return (
+    <div
+      className={["sui-panelhdr", className ?? ""].filter(Boolean).join(" ")}
+    >
+      <div className="sui-panelhdr__left">
+        {onBack && (
+          <button
+            type="button"
+            className="sui-panelhdr__back"
+            onClick={onBack}
+            aria-label="Back"
+          >
+            <svg
+              viewBox="0 0 24 24"
+              width={18}
+              height={18}
+              fill="none"
+              stroke="currentColor"
+              strokeWidth={1.75}
+              strokeLinecap="round"
+              strokeLinejoin="round"
+            >
+              <polyline points="15 18 9 12 15 6" />
+            </svg>
+          </button>
+        )}
+        <div className="sui-panelhdr__text">
+          <div className="sui-panelhdr__title">{title}</div>
+          {subtitle && <div className="sui-panelhdr__sub">{subtitle}</div>}
+        </div>
+      </div>
+      {actions && <div className="sui-panelhdr__actions">{actions}</div>}
+    </div>
+  );
+}