Make headless server the default and gate desktop mode

[teloverge]

What changed

• Make the headless server the default application mode and gate the Tauri desktop shell behind the desktop feature.
• Configure Cargo/Tauri Windows packaging to select the dedicated stellr-desktop binary when multiple entry points exist.
• Add authenticated Tailnet serving and cross-platform Tailnet URL helper scripts.
• Update authentication, CI/release configuration, frontend controls, documentation, and changelog entries for the new mode.

Why

This enables remote browser access over a Tailnet while ensuring Windows Tauri packages launch the console-free desktop binary. Headless documentation now explicitly selects the stellr CLI binary.

Validation

• Reproduced the Windows packaging assertion locally: it failed before the manifest change and passed after it.
• cargo fmt --check passed.
• cargo metadata --no-deps --format-version 1 passed.
• cargo test --workspace --locked passed.
• npm --prefix web run check passed: 0 errors and 0 warnings.
• npm --prefix web test passed: 33 files, 278 tests.
• npm --prefix web run build passed.
• The full PowerShell packaging/build smoke remains GitHub Windows-runner validation; this environment has no Windows PowerShell host.