feat: add storage info to settings page and improve NFS export handling

Backend:
- Add source/used_by/available/error fields to diskUsage in system status
- collectDiskUsage() now reads /proc/mounts, samba shares and NFS exports from DB
- Paths are deduplicated; shared paths list all shares/exports using them
- syscall.Statfs errors surface as available=false with user-facing error
- collectServiceStatus made a method of Server (receiver consistency)

Frontend:
- Settings page now shows two cards: mount points and shared resources
- Each path shows source badge (Sistema/Mount/SMB/NFS), used_by chips, progress bar
- Unavailable paths show amber warning instead of progress bar
- DiskUsage interface updated with new fields
- NFSExport interface updated with structured fields (fsid, async, etc)
- NFS page updated to use new export fields

internal/db/migrations/0002_nfs_structured.sql

@@ -0,0 +1,9 @@
+ALTER TABLE nfs_exports
+    ADD COLUMN read_only     INTEGER NOT NULL DEFAULT 0,
+    ADD COLUMN async_       INTEGER NOT NULL DEFAULT 0,
+    ADD COLUMN root_squash   INTEGER NOT NULL DEFAULT 1,
+    ADD COLUMN subtree_check INTEGER NOT NULL DEFAULT 0,
+    ADD COLUMN fsid         INTEGER NOT NULL DEFAULT 0,
+    ADD COLUMN advanced      TEXT NOT NULL DEFAULT '{}';
+
+CREATE INDEX IF NOT EXISTS idx_nfs_exports_fsid ON nfs_exports(fsid);