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

Makefile

@@ -1,5 +1,5 @@
 BINARY=nasctl
-VERSION?=0.2.1
+VERSION?=0.3.0
 GO?=go
 LDFLAGS=-s -w -X github.com/darroyo/nasctl/internal/web.Version=$(VERSION) -X github.com/darroyo/nasctl/internal/web.Commit=$(shell git rev-parse --short HEAD 2>/dev/null || echo unknown)
 BUILD_FLAGS=CGO_ENABLED=0