feat: multi-source disk entries with chips display

diskUsage now carries Sources []string instead of a single Source.
collectDiskUsage accumulates all sources (samba, nfs, manual) per path.
NFS no longer adds a redundant label to used_by.

Dashboard and Settings render one chip per source. When a path
is shared via SMB and NFS, both chips appear.

API breaking: disks[].source replaced by disks[].sources[].
Version: 0.2.1 -> 0.3.0

web/src/api.ts

@@ -48,7 +48,7 @@ export interface DiskUsage {
   free_bytes: number;
   used_bytes: number;
   used_percent: number;
-  source: "system" | "mount" | "samba" | "nfs" | "manual";
+  sources: ("manual" | "samba" | "nfs")[];
   used_by?: string[];
   available: boolean;
   error?: string;