R-108: network storage may not host an app's data namespace (v0.187.0)

This is D5's precondition and it is now met.

An app's namespace root IS its backup root: namespaceRoot returns a non-system
drive path as-is, so the recovery unit lands at <HDD_PATH>/backups/primary/<stack>/.
On a NAS that sits inside the share, which FileBrowser binds WHOLE — share root,
:rslave, download:true.

The bind was NOT narrowed, and establishing why inverted the fix. The share-root
:rslave bind is load-bearing (a 2026-07-22 probe proved an in-container access
through it wakes the idle automount trigger), and scoping is undefinable anyway:
apps on a share store at <share>/<app>, there is no userdata/ layer, and creating
one would write Felhom convention onto a customer's own NAS, which R-67 forbids.
So the browsing surface cannot be narrowed and the backup tree must never be
placed under it. Operator ruling: refuse the placement, keep the browse bind.
Tier 2 already refuses network targets for this reason (F-6C-1).

Nothing stranded: zero apps on network storage across all six hub customers
including Peti. R-67's browse capability is byte-identical.

FIVE surfaces, not the four the register named — settings.RefuseAsAppNamespace is
the single predicate. The deploy POST is the real boundary (it accepts any
caller-supplied HDD_PATH; DeployStack validates only os.Stat). Surface 4,
handleStorageDecommission mode=migrate, guarded only its SOURCE, so a whole
namespace could be decommissioned ONTO a NAS — that one is not in the register.

Fails closed: /mnt/felhom-drives holds both kinds, Kind exists only on a
registered path, so an unregistered path under that root refuses.

Supersedes README's "NAS backup locality — decision A" (v0.118.0).

9 tests, all non-effect (nil stackMgr, so a guard that misses panics rather than
passing). 4 red-proofs, each mutation asserted to have landed.
Suite rc=0, 27 packages, 0 FAIL. vet rc=0. Template + emoji gates OK.

controller/internal/web/templates/deploy.html

@@ -565,8 +565,9 @@
                         onchange="checkStorageSpace(this)">
                     {{range $.StoragePaths}}
                     <option value="{{.Path}}" data-free-percent="{{printf "%.0f" .FreePercent}}"
-                            {{if $.AlreadyDeployed}}{{if eq .Path $.CurrentHDDPath}}selected{{end}}{{else if .IsDefault}}selected{{end}}>
-                        {{.Label}} — {{.FreeHuman}} szabad{{if .IsDefault}} (alapértelmezett){{end}}
+                            {{if .NotAllowed}}disabled{{end}}
+                            {{if $.AlreadyDeployed}}{{if eq .Path $.CurrentHDDPath}}selected{{end}}{{else if and .IsDefault (not .NotAllowed)}}selected{{end}}>
+                        {{.Label}} — {{.FreeHuman}} szabad{{if .NotAllowed}} ({{.NotAllowedNote}}){{else if .IsDefault}} (alapértelmezett){{end}}
                     </option>
                     {{end}}
                     {{if $.CurrentHDDPathMissing}}