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/webui/src/index.html

@@ -13,7 +13,7 @@
        on the asset handlers, the practical caching window is one
        version. -->
   <link rel="stylesheet" href="/assets/app.css?v={{ASSET_VERSION}}" />
-  <link rel="icon" type="image/svg+xml" href="/assets/logo.svg?v={{ASSET_VERSION}}" />
+  <link rel="icon" type="image/svg+xml" href="/assets/logo.svg?v={{ASSET_VERSION}}&r={{LOGO_REV}}" />
   <!-- Theme is read from localStorage *before* paint to avoid the
        dark→light flash on every navigation. Falls back to the OS
        preference and finally to dark. -->
@@ -33,7 +33,7 @@
 <div class="shell">
   <aside class="sidebar">
     <div class="brand">
-      <img src="/assets/logo.svg?v={{ASSET_VERSION}}" alt="OpenPXE" />
+      <img src="/assets/logo.svg?v={{ASSET_VERSION}}&r={{LOGO_REV}}" alt="OpenPXE" />
       <strong>OpenPXE</strong>
     </div>
     <nav>