feat: structured NFS options with auto-fsid and manual watched mounts

NFS exports: replace plain-text options string with typed booleans
(read_only, async, root_squash, subtree_check) + advanced JSON blob.
fsid is auto-generated via crypto/rand with collision retry and is
never user-settable. Breaking API change (options field removed).

Dashboard: filter disks to manual+samba+nfs sources only; no more
auto-discovery of all /proc/mounts entries.

Settings: new 'Puntos de montaje vigilados' card with add/remove
for manual mount points. AllowedRoots validation applied.

Version bump: 0.1.10 -> 0.2.0

Makefile

@@ -1,5 +1,5 @@
 BINARY=nasctl
-VERSION?=0.1.10
+VERSION?=0.2.0
 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