Report the application version from the canonical build value

[debpalash]

Problem

build.zig.zon declares version 0.6.5, while GET /api/host in src/services/remote.zig returns a hard-coded 0.1.2. Other call sites also embed release-looking strings.

Acceptance criteria

• /api/host uses the version injected from build.zig.zon through build options.
• User agent/version strings that represent the application release use the same source.
• No application version literal remains outside the canonical metadata, tests, fixtures, or historical changelog.
• Add a unit/feature test comparing host API output with the canonical version.
• Document how development builds are represented.