v0.4.68: fix NFS secure-export mount, logo cache-bust, dashboard disk card, NFS form spacing

Four operator-reported issues from v0.4.67 validation.

## 1. NFS MNT3ERR_ACCES even with the host IP allow-listed

Root cause: Linux kernel nfsd (what UniFi UNAS / Synology / TrueNAS all
run underneath) exports with the `secure` option by default, which only
accepts mount/NFS requests from a privileged source port (<1024). v0.4.67
explicitly connected from a non-privileged port on the mistaken assumption
that uid 10001 can't bind low ports — but the binary carries
CAP_NET_BIND_SERVICE (granted via setcap for the DHCP/TFTP/HTTP low-port
binds), which also covers privileged *source* ports for outbound connects.

Fix: build_connection now tries a privileged source port first (the common
case for every appliance NAS), then falls back to a non-privileged port
for `insecure` exports or capability-less environments. Each attempt has
its own connect timeout; a timeout on the first attempt skips the fallback
(the server isn't answering — a retry would just double the wait).

Also: hint_for now recognizes MNT3ERR_ACCES distinctly from NFS3ERR_ACCES
and explains both the allow-list and the secure/insecure angle, with the
UniFi /var/nfs/shared/<share> path convention called out.

## 2. Custom logo didn't update the top-left brand mark

The brand <img> and favicon were pinned to ?v=<app-version>, which only
changes on upgrade — so uploading a new logo left the cached bundled SVG
in place. Added a monotonic `rev` counter to BrandingStore that bumps on
every set/clear, persisted across restarts, surfaced through index_html as
an extra &r=<rev> cache-bust token on the brand mark + favicon URLs. Since
index.html is served no-cache, the fresh token lands on the next reload
after upload and the new logo appears immediately.

(Note: this updates the WebUI brand mark. The PXE *boot menu* still shows
the ASCII wordmark — painting the operator's PNG there needs the
IMAGE_PNG-enabled iPXE rebuild that remains queued for native x86_64
hardware. The /branding/pxe-logo compositor is ready for when it lands.)

## 3. Disk-space card on the Dashboard

Extracted the Storage tab's disk card into a shared diskSpaceCard(disk)
helper and added it to the Dashboard grid under the stat strip. Dashboard
fetches /api/storage/disk with the same graceful-degradation fallback the
Storage tab uses.

## 4. NFS "Add share" button touching the form field

The NFS card has a single form row (vs SMB's two), so the button butted
right against it. Added margin-top:14px to match SMB's effective spacing.

Tests: 162 passing (+2 — logo_rev bump, MNT3ERR_ACCES hint). clippy clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

crates/core/src/branding.rs

@@ -44,6 +44,14 @@ struct Inner {
     /// MIME of the active logo, mirroring `logo_filename`. Cached here
     /// so the HTTP layer can set Content-Type without re-sniffing.
     logo_mime: Option<String>,
+    /// Monotonic counter bumped on every set/clear. Surfaces as a
+    /// cache-bust token (`/assets/logo.svg?r=<rev>`) so the browser
+    /// fetches the new bytes the moment the operator swaps the logo —
+    /// the app version alone can't do this since it doesn't change on
+    /// upload. Persisted so the token stays stable across restarts and
+    /// keeps climbing across multiple swaps.
+    #[serde(default)]
+    rev: u64,
 }
 
 /// In-memory + on-disk override registry. Cheap to clone; locks are
@@ -150,6 +158,7 @@ impl BrandingStore {
             let mut g = self.inner.write();
             g.logo_filename = Some(filename.clone());
             g.logo_mime = Some(mime.to_string());
+            g.rev = g.rev.wrapping_add(1);
         }
         self.persist();
         tracing::info!(
@@ -166,6 +175,7 @@ impl BrandingStore {
             let mut g = self.inner.write();
             let removed = g.logo_filename.take();
             g.logo_mime = None;
+            g.rev = g.rev.wrapping_add(1);
             removed
         };
         if let Some(name) = removed {
@@ -185,6 +195,14 @@ impl BrandingStore {
         self.inner.read().logo_filename.is_some()
     }
 
+    /// Cache-bust token for the logo asset URL. Changes on every
+    /// set/clear so `/assets/logo.svg?r=<rev>` resolves to a fresh URL
+    /// whenever the operator swaps the brand mark. Stable otherwise.
+    #[must_use]
+    pub fn logo_rev(&self) -> u64 {
+        self.inner.read().rev
+    }
+
     fn persist(&self) {
         let snap = self.inner.read().clone();
         let body = match serde_json::to_vec_pretty(&snap) {
@@ -292,6 +310,23 @@ mod tests {
         assert!(!entries.iter().any(|n| n == "logo.png"), "stale PNG left over: {entries:?}");
     }
 
+    #[test]
+    fn logo_rev_bumps_on_each_set_and_clear() {
+        let dir = tempdir().unwrap();
+        let b = BrandingStore::load_or_default(dir.path());
+        assert_eq!(b.logo_rev(), 0);
+        b.set_logo("image/png", "png", b"\x89PNG\r\n\x1a\nfake").unwrap();
+        assert_eq!(b.logo_rev(), 1);
+        b.set_logo("image/png", "png", b"\x89PNG\r\n\x1a\nfake2").unwrap();
+        assert_eq!(b.logo_rev(), 2);
+        b.clear_logo().unwrap();
+        assert_eq!(b.logo_rev(), 3);
+        // Survives a restart.
+        drop(b);
+        let b2 = BrandingStore::load_or_default(dir.path());
+        assert_eq!(b2.logo_rev(), 3);
+    }
+
     #[test]
     fn sanitize_ext_strips_separators_and_path_chars() {
         assert_eq!(sanitize_ext("svg"), "svg");
@@ -319,6 +354,7 @@ mod tests {
         let inner = Inner {
             logo_filename: Some("logo.png".into()),
             logo_mime: Some("image/png".into()),
+            rev: 0,
         };
         std::fs::write(
             brand_dir.join("branding.json"),