Linux GTK port of the Host Manager (DESIGN.md) Phase 7

[gitaman69]

I would like to implement the Linux/GTK port of the Sarv feature layer, as
outlined in macos/Sources/Features/HostManager/DESIGN.md (Phase 7): feature
logic reimplemented in the GTK apprt, with the on-disk formats
(hosts.json, vault files) as the cross-platform contract, and libsecret in
place of the macOS Keychain.

Questions

1. Is Phase 7 still the intended architecture, or should shared logic be
   extracted into the Zig core instead?
2. Is anyone already working on this?
3. Can this start now, or must it wait until the macOS data formats are
   considered stable?

Proposed first PR

Read-only host sidebar in the GTK apprt:

• New class: src/apprt/gtk/class/host_sidebar.zig
• Parses ~/.config/sarvterminal/hosts.json; click opens a tab running ssh <host>
• Adds toggle_host_sidebar / focus_host_sidebar_search per DESIGN.md 11
  (appended to the Key enum to preserve ABI)