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.

src/types/desktop/maintenanceTypes.ts

@@ -1,50 +0,0 @@
-import type { PrimeVueSeverity } from '../primeVueTypes'
-
-interface MaintenanceTaskButton {
-  /** The text to display on the button. */
-  text?: string
-  /** CSS classes used for the button icon, e.g. 'pi pi-external-link' */
-  icon?: string
-}
-
-/** A maintenance task, used by the maintenance page. */
-export interface MaintenanceTask {
-  /** ID string used as i18n key */
-  id: string
-  /** The display name of the task, e.g. Git */
-  name: string
-  /** Short description of the task. */
-  shortDescription?: string
-  /** Description of the task when it is in an error state. */
-  errorDescription?: string
-  /** Description of the task when it is in a warning state. */
-  warningDescription?: string
-  /** Full description of the task when it is in an OK state. */
-  description?: string
-  /** URL to the image to show in card mode. */
-  headerImg?: string
-  /** The button to display on the task card / list item. */
-  button?: MaintenanceTaskButton
-  /** Whether to show a confirmation dialog before running the task. */
-  requireConfirm?: boolean
-  /** The text to display in the confirmation dialog. */
-  confirmText?: string
-  /** Called by onClick to run the actual task. */
-  execute: (args?: unknown[]) => boolean | Promise<boolean>
-  /** Show the button with `severity="danger"` */
-  severity?: PrimeVueSeverity
-  /** Whether this task should display the terminal window when run. */
-  usesTerminal?: boolean
-  /** If `true`, successful completion of this task will refresh install validation and automatically continue if successful. */
-  isInstallationFix?: boolean
-}
-
-/** The filter options for the maintenance task list. */
-export interface MaintenanceFilter {
-  /** CSS classes used for the filter button icon, e.g. 'pi pi-cross' */
-  icon: string
-  /** The text to display on the filter button. */
-  value: string
-  /** The tasks to display when this filter is selected. */
-  tasks: ReadonlyArray<MaintenanceTask>
-}

src/types/primeVueTypes.ts

@@ -1,10 +0,0 @@
-/** Button, Tag, etc severity type is 'string' instead of this list. */
-export type PrimeVueSeverity =
-  | 'primary'
-  | 'secondary'
-  | 'success'
-  | 'info'
-  | 'warn'
-  | 'help'
-  | 'danger'
-  | 'contrast'