Make Manual Backup match the system role (Desktop Only scope)

The Manual Backup screen in the Hub always listed the Node / Server +
Desktop items (nix-bitcoin secrets, /var/lib system service data, and the
database/blockchain caveat), and the backup script mirrored /var/lib and
counted it in the free-space estimate even on the Desktop Only role.
Desktop Only systems run no server or Bitcoin services and have no
internal second data drive, so none of that applies.

Hub UI (support.js):
- 'What gets backed up' is role-aware: Desktop Only lists only the NixOS
  configuration (/etc/nixos) and home directory (/home)
- Database/blockchain note hidden on Desktop Only
- Intro copy corrected: external USB copy is a second location on
  Desktop Only (no internal second drive); third-location wording kept
  for Node / Server + Desktop

Backup script (sovran-hub-backup.sh):
- Desktop Only runs 2 stages (1/2 /etc/nixos, 2/2 /home); secrets and
  /var/lib stages no longer run on that role
- Free-space estimate skips /var/lib on Desktop Only
- BACKUP_MANIFEST.txt sources/exclusions/limitations/restore guidance
  and blockchain note are role-aware
- Completion message role-aware ('second, external location' on
  Desktop Only); header comments updated

Node and Server + Desktop behavior is unchanged. Added CHANGELOG entry.

Verified: bash -n / node --check, 6 role-detection cases, manifest
generation for both role groups (non-desktop output identical to
before), and simulated UI renders for all three roles.

Co-authored-by: arena-agent <297053741+arena-agent@users.noreply.github.com>

CHANGELOG.md

@@ -7,6 +7,19 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ---
 
+## [Unreleased]
+
+### Fixed
+- **Manual Backup now matches the system role**: on the Desktop Only role the Hub's
+  "What gets backed up" list no longer shows Node / Server + Desktop items
+  (nix-bitcoin secrets, `/var/lib` system service data, and the database/blockchain
+  caveats), and the backup script skips those stages entirely — Desktop Only backups
+  now mirror only the NixOS configuration (`/etc/nixos`) and home directory (`/home`).
+  The free-space estimate, stage numbering, backup manifest, and completion message
+  are all role-aware; Node and Server + Desktop backups are unchanged.
+
+---
+
 ## [1.0.4] - 2026-07-29
 
 ### Added