Decouple Desktop UI into monorepo app (#5912)

## Summary

Extracts desktop UI into apps/desktop-ui package with minimal changes.

## Changes

- **What**:
- Separates desktop-specific code into standalone package with
independent Vite config, router, and i18n
- Drastically simplifies the main app router by removing all desktop
routes
  - Adds a some code duplication, most due to the existing design
- Some duplication can be refactored to be *simpler* on either side - no
need to split things by `isElectron()`
  - Rudimentary storybook support has been added
- **Breaking**: Stacked PR for publishing must be merged before this PR
makes it to stable core (but publishing _could_ be done manually)
  - #5915
- **Dependencies**: Takes full advantage of pnpm catalog. No additional
dependencies added.

## Review Focus

- Should be no changes to normal frontend operation
- Scripts added to root package.json are acceptable
- The duplication in this PR is copied as is, wherever possible. Any
corrections or fix-ups beyond the scope of simply migrating the
functionality as-is, can be addressed in later PRs. That said, if any
changes are made, it instantly becomes more difficult to separate the
duplicated code out into a shared utility.
  - Tracking issue to address concerns: #5925

### i18n

Fixing i18n is out of scope for this PR. It is a larger task that we
should consider carefully and implement properly. Attempting to isolate
the desktop i18n and duplicate the _current_ localisation scripts would
be wasted energy.

package.json

@@ -8,10 +8,12 @@
   "description": "Official front-end implementation of ComfyUI",
   "license": "GPL-3.0-only",
   "scripts": {
+    "build:desktop": "nx build @comfyorg/desktop-ui",
     "build-storybook": "storybook build",
     "build:types": "nx build --config vite.types.config.mts && node scripts/prepare-types.js",
     "build": "pnpm typecheck && nx build",
     "collect-i18n": "pnpm exec playwright test --config=playwright.i18n.config.ts",
+    "dev:desktop": "nx dev @comfyorg/desktop-ui",
     "dev:electron": "nx serve --config vite.electron.config.mts",
     "dev": "nx serve",
     "devtools:pycheck": "python3 -m compileall -q tools/devtools",