v0.172.0 — R-75: canonical import root, catalog-derived skeleton, import surfaces

${IMPORT_PATH} = <system namespace root>/userdata/import — ONE drop-zone per box,
on the system drive, injected at BOTH compose-env builders with NO per-drive
fallback (unresolvable leaves it unset so compose fails loudly rather than
quietly building a second, dead drop-zone).

Third BindRoot (RootImport) + Import list in BackupSpec, extended through
ValidateBackupSpec/ClassifyBinds. Load-bearing: a stale `userdata: import/<app>`
entry against the moved bind would be a WHOLE-BLOCK reject, taking the app's
mandatory hdd classification with it.

Exhaustive-root audit: resolveAbs/structuralGuard/ComputeCaptureSet/
ComputeFabBuckets now take importRoot explicitly (an import bind resolved
against hddPath would name a directory on the wrong drive); unresolvable is
refused loudly into Skipped. GetImportRoot added to both provider interfaces.

Catalog-derived skeleton: UserdataSkeleton() -> UserdataSkeletonCarry() +
BuildUserdataSkeleton(), SORTED. The carry-list makes zero-removals true by
construction (`documents` is in no catalog app but on both boxes) and is the
fresh-box floor. The sort is not tidiness: the naive map-order derivation
measured 20 distinct outputs from 20 identical runs, which with fbNeedsRecreate
is a fleet-wide FileBrowser restart loop.

One authoritative compose parser: ParseComposeUserdataMounts now delegates to
ParseComposeClassifiableBinds. Import root excluded from per-app migration.

Surfaces: FileBrowser /srv/beolvasas source; app-page "Hova tegyem a fajlokat?"
with PathEscape deep links (never QueryEscape) and class-driven copy;
data_paths: annotation with the Fork-3 asymmetry; system-owned beolvasas SMB
share refused server-side at handler AND store, button omitted in template.

Caught on the way: the sharing template's row struct was function-local, so
adding {{if .System}} would have 500'd every share row. ShareRow is now
package-level and the render test uses the handler's own type.

Tests 915 -> 949, all green. MinAgent unchanged.

controller/internal/appbackup/appdata.go

@@ -19,7 +19,11 @@ type StackDataProvider interface {
 	GetStackComposePath(name string) (composePath string, ok bool)
 	ListDeployedStacks() []StackSummary
 	GetStackHDDMounts(name string) []string
-	GetStackHDDPath(name string) string    // raw HDD_PATH from app.yaml (empty if no HDD)
+	GetStackHDDPath(name string) string // raw HDD_PATH from app.yaml (empty if no HDD)
+	// GetImportRoot returns the CANONICAL drop-zone root (R-75): <system namespace root>/userdata/import.
+	// It is app-INDEPENDENT and lives on the SYSTEM drive, so ${IMPORT_PATH} binds cannot be resolved
+	// from GetStackHDDPath. Empty when unresolvable — structuralGuard refuses such binds loudly.
+	GetImportRoot() string
 	GetDockerVolumes(name string) []string // full Docker volume names (project-prefixed)
 	StopStack(name string) error
 	StartStack(name string) error