v0.4.4: Settings tab, API reference, ISO category, branding, disk space

Settings:
- New top-level Settings tab. Carries a placeholder for the planned
  LDAP / OIDC / user-management work, the new branding controls, and
  the API reference at the bottom.
- Custom logo upload (PNG/SVG/JPEG/WebP/GIF up to 2 MB) replaces the
  bundled brand mark via /assets/logo.svg; bytes live at
  <work_dir>/branding/ and survive restart. The original "OpenPXE
  v<x.y.z>" pins to the sidebar footer for support.
- API reference rendered from a new GET /api/docs into a per-method
  coloured pill list grouped by area.

ISO category (Storage):
- New IsoCategory { Os, Tools } on IsoMeta with PUT
  /api/isos/:id/category. Storage table's Type cell becomes a
  dropdown; selecting Tools moves the ISO into the Tools submenu next
  to memtest / shell / NIC info and removes it from the OS Installers
  family submenu. Family detection still drives BIOS/UEFI / kernel
  args; only the menu placement changes.

Storage telemetry:
- New IsoStore::disk_usage (libc::statvfs, lives in iso-store so the
  http-api crate stays #![forbid(unsafe_code)]) and GET
  /api/storage/disk. The Storage tab now shows free/used/total for
  the volume hosting the ISO directory with an 80%/95% colour ramp.

UI polish:
- Brand block in the sidebar now matches the topbar height exactly,
  so the divider runs straight across the top of the app rather than
  stepping; version label moved out of the brand and pinned to the
  sidebar footer ("OpenPXE v0.4.4").
- Light-mode terminal: --terminal-bg + per-level text colours track
  the active theme rather than being hard-coded dark.
- About: lead paragraph spans the full content width; new Docs row
  links to https://openpxe.com/.

106 tests passing (was 89 in v0.4.1, +17 across branding unit tests
and new integration coverage for category / disk / docs / branding).
cargo clippy --workspace --all-targets clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

crates/http-api/src/state.rs

@@ -1,6 +1,7 @@
 use crate::uploads::UploadSessions;
 use openpxe_core::{
-    BootLog, ClientRegistry, DeploymentQueue, HostBindings, LogBus, Metrics, SettingsStore,
+    BootLog, BrandingStore, ClientRegistry, DeploymentQueue, HostBindings, LogBus, Metrics,
+    SettingsStore,
 };
 use openpxe_iso_store::{IsoStore, NfsManager, SmbManager};
 use std::sync::Arc;
@@ -20,6 +21,10 @@ pub struct AppState {
     /// every `/boot/<entry>.ipxe` chain that goes on to serve a script
     /// (i.e. an image actually starting to install on a machine).
     pub boot_log: BootLog,
+    /// Operator-controlled UI overrides (custom logo). When the
+    /// operator hasn't uploaded anything, the WebUI serves the bundled
+    /// rainbow-horizon mark.
+    pub branding: BrandingStore,
     /// Lock-free metrics counters surfaced at `/metrics` in Prometheus
     /// text format. Cheap to clone (handles to atomics).
     pub metrics: Metrics,