feat: network topology SVG diagram for v1.8.0

[franklupo]

Summary

• Every export now produces a standalone SVG network topology diagram next to the Excel file (same basename). For each Proxmox node it shows the full chain NICs → bonds → bridges → gateway VMs → internal bridges → leaf VMs, plus a dedicated strip below for network-backed storage (NFS, CIFS, PBS, iSCSI, Ceph, RBD, GlusterFS). Multi-homed VMs (potential gateways) and inactive interfaces get distinct colours so routing is visible at a glance.
• More resilient QEMU agent collection — each agent sub-call (get-osinfo, get-host-name, network-get-interfaces, get-fsinfo) is now awaited independently. A single failure (e.g. OPNsense returning an error object on get-fsinfo) no longer wipes hostname / OS / network info for the VM.
• Clearer agent error messages in the Hostname column when the agent throws.
• New helpers (TaskExtensions.WhenAllSafe, Task<T>.ResultOrDefault(), LabeledValue, VmTypeLabel, SplitWords, MtuLabel, ActiveStatus, YesNo, InterfaceTitle, BoxLabel, RenderBox, AddNicNode) consolidate repeated patterns across ReportEngine.*.
• New docs page docs/network-diagram.md, anonymised sample docs/network-diagram.svg, README updated with diagram preview.
• Storage /storage API call now made once in LoadResourcesAsync and reused by both the Storages sheet and the diagram.
• Bumps version to 1.8.0 and adds a changelog entry.

Test plan

• cv4pve-report @config.rsp export produces both Report_*.xlsx and Report_*.svg
• cv4pve-report @config.rsp export -o /tmp/myreport.xlsx produces /tmp/myreport.svg alongside the xlsx
• Open the SVG in a browser: legend, info box (counts), per-node chain and storage strip render correctly
• Multi-homed VMs (e.g. OPNsense + storage hosts) appear orange; stopped VMs and inactive interfaces appear grey; storages with a gateway on the host show their bridge arrow correctly
• OPNsense / FreeBSD VMs report a real hostname (no longer "Agent not running!" because of get-fsinfo)
• Excel report unchanged: cover, all sheets, hyperlinks still work
• Fast / Standard / Full profiles still generate without errors