Desktop: Fallback to local backend if self-hosted server is offline (#5880)

* Adds a fallback mechanism so the desktop app routes tool operations to
the local bundled backend when the user's self-hosted Stirling-PDF
server goes offline, and disables tools in the UI that aren't supported
locally.

* `selfHostedServerMonitor.ts` independently polls the self-hosted
server every 15s and exposes which tool endpoints are unavailable when
it goes offline
* `operationRouter.ts` intercepts operations destined for the
self-hosted server and reroutes them to the local bundled backend when
the monitor reports it offline
* `useSelfHostedToolAvailability.ts` feeds the offline tool set into
useToolManagement, disabling affected tools in the UI with a
selfHostedOffline reason and banner warning

- `SelfHostedOfflineBanner `is a dismissable (session-only) gray bar
shown at the top of the UI when in self-hosted mode and the server goes
offline. It shows:

frontend/public/locales/en-GB/translation.toml

@@ -5726,6 +5726,40 @@ placeholder = "Enter search term..."
 searching = "Searching..."
 title = "Search PDF"
 
+[selfHosted]
+
+[selfHosted.offline]
+hideTools = "Hide unavailable tools ▴"
+messageNoFallback = "Tools are unavailable until your server comes back online."
+messageWithFallback = "Some tools require a server connection."
+showTools = "View unavailable tools ▾"
+title = "Your Stirling-PDF server is unreachable"
+toolNotAvailableLocally = "Your Stirling-PDF server is offline and \"{{endpoint}}\" is not available on the local backend."
+
+[selfHosted.offline.conversionTypes]
+cbr-to-pdf = "CBR → PDF"
+cbz-to-pdf = "CBZ → PDF"
+eml-to-pdf = "Email → PDF"
+ebook-to-pdf = "eBook → PDF"
+file-to-pdf = "Office/Document → PDF"
+html-to-pdf = "HTML → PDF"
+img-to-pdf = "Image → PDF"
+markdown-to-pdf = "Markdown → PDF"
+pdf-to-cbr = "PDF → CBR"
+pdf-to-cbz = "PDF → CBZ"
+pdf-to-csv = "PDF → CSV"
+pdf-to-epub = "PDF → EPUB"
+pdf-to-html = "PDF → HTML"
+pdf-to-img = "PDF → Image"
+pdf-to-markdown = "PDF → Markdown"
+pdf-to-pdfa = "PDF → PDF/A"
+pdf-to-presentation = "PDF → Presentation"
+pdf-to-text = "PDF → Text"
+pdf-to-word = "PDF → Word"
+pdf-to-xml = "PDF → XML"
+pdf-to-xlsx = "PDF → XLSX"
+svg-to-pdf = "SVG → PDF"
+
 [session]
 expired = "Your session has expired. Please refresh the page and try again."
 refreshPage = "Refresh Page"
@@ -6531,6 +6565,7 @@ favorites = "Favourites"
 heading = "All tools (fullscreen view)"
 noResults = "Try adjusting your search or toggle descriptions to find what you need."
 recommended = "Recommended"
+selfHostedOffline = "Requires your Stirling-PDF server (currently offline):"
 showDetails = "Show Details"
 unavailable = "Disabled by server administrator:"
 unavailableDependency = "Unavailable - required tool missing on server:"