applications: fix relative installer/link hrefs + serve /installers via IIS

Two problems with application download/launch/doc links:

1. Stored paths like 'installers/Foo.exe' are relative, so an <a href> on
   /shopdb/applications/6 resolved to /shopdb/applications/installers/Foo.exe.
   New basePath.fileHref() mounts a relative path under the app base
   (-> /shopdb/installers/Foo.exe) while leaving full URLs and UNC/file paths
   untouched. Applied to installpath, applicationlink, and documentationpath in
   the list and detail views.

2. Even the correct /shopdb/installers/Foo.exe 404s: httpPlatformHandler is
   path="*", so IIS forwards it to Flask, which has no such route. Add a
   web.config <location path="installers"> that clears the handler and serves
   that subpath as IIS static (with .exe/.msi MIME), from a physical
   APP_ROOT\installers folder.

frontend/src/utils/basePath.js

@@ -12,6 +12,24 @@ export function withBase(path) {
   return BASE_URL + String(path).replace(/^\//, '')
 }
 
+// Resolve a stored file/link path for use in an <a href>. Full URLs (http,
+// https, file:, mailto:, ...) and UNC / protocol-relative paths (\\server,
+// //host) open as-is. A path relative to the app (e.g. 'installers/x.exe' or
+// './installers/x.exe') is mounted under the base so it does NOT resolve
+// against the current route - '/shopdb/applications/6' + 'installers/x.exe'
+// wrongly became '/shopdb/applications/installers/x.exe'.
+export function fileHref(path) {
+  if (!path) return null
+  const trimmed = String(path).trim()
+  if (!trimmed) return null
+  if (/^[a-z][a-z0-9+.-]*:/i.test(trimmed)
+      || trimmed.startsWith('\\\\')
+      || trimmed.startsWith('//')) {
+    return trimmed
+  }
+  return withBase('/' + trimmed.replace(/^(\.?\/)+/, ''))
+}
+
 // Inverse of withBase: turn a browser pathname (which includes the mount
 // base, e.g. '/ops/computers') into a router path ('/computers'). Router
 // navigation already applies the base; feeding it an un-stripped pathname