v0.15.0: Attach existing drive wizard (bind mount, no format)

New Settings wizard to attach drives with existing filesystems without
formatting. Mounts partition at staging path, lets user browse and pick
a subfolder, then bind-mounts it at /mnt/<name> with fstab entries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/storage/attach.go

@@ -0,0 +1,22 @@
+package storage
+
+// AttachRequest holds parameters for attaching an existing partition
+// without formatting — using a bind mount from a subfolder.
+type AttachRequest struct {
+	DevicePath string // "/dev/sdb1" — must have an existing filesystem
+	MountName  string // "hdd_1" → bind-mounts at /mnt/hdd_1
+	SubPath    string // full path on raw mount to bind-mount (e.g., "/mnt/.felhom-raw/hdd_1/felhom_data")
+	Label      string // Display label for the UI
+	SetDefault bool   // Register as default storage path
+}
+
+// DirEntry represents a directory entry returned by ListDirectories.
+type DirEntry struct {
+	Name        string `json:"name"`
+	Path        string `json:"path"`
+	HasChildren bool   `json:"has_children"`
+}
+
+// RawMountBase is the hidden staging directory where partitions are
+// temporarily mounted for browsing before the final bind mount.
+const RawMountBase = "/mnt/.felhom-raw"