Add channel tui option

[r0x0r]

This pull request adds support for selecting the ROCm SDK installation channel (release or nightly) in both the onboarding and install manager flows of the TUI. Users can now switch between stable (release) and preview (nightly) channels via the UI, and the selected channel is used when installing the SDK. The UI, keyboard handling, and test code have all been updated to support this new capability.

[rominf]

Review

Thanks for the PR — the toggle semantics, Release default, and correct threading of the channel value through to install sdk --channel are all sound. Tests are well updated and replacing handle_key(Up) navigation with direct select_onboarding_choice() calls is a clear improvement.

A few issues need addressing before merge.

Critical: channel label always shows "Release"

Both install_sdk_choice_label and onboarding_menu_label hardcode the string "Release" as the left-side label regardless of the selected channel:

InstallSdkChoice::Channel => format!(
    "{:<14} {}",
    "Release",        // ← always "Release"
    if matches!(channel, InstallSdkChannel::Release) {
        "Stable (default)"
    } else {
        "Nightly (preview)"  // when nightly: displays "Release         Nightly (preview)"
    }
),

When the user switches to Nightly the row reads Release Nightly (preview) — contradictory. Compare with the Folder row: Install folder: /path/. The channel row label should be a neutral descriptor like "Channel:" so it stays coherent in both states: Channel: Nightly (preview).

toggle_install_sdk_channel updates status unconditionally

fn toggle_install_sdk_channel(&mut self) {
    if let Some(state) = self.install_manager.as_mut()
        && let InstallManagerScreen::Sdk { channel, .. } = &mut state.screen
    {
        *channel = channel.toggle();
        state.message = None;
        state.detail_scroll = 0;
    }
    self.status = "ROCm channel changed. ...".to_owned(); // ← runs even when the if let fails
}

If the install manager isn't on the SDK screen, the toggle silently does nothing but the status still claims the channel changed. Move self.status = inside the if let block.

Duplicate Left/Right arms in handle_onboarding_key

The Left and Right handlers for OnboardingMenuChoice::Channel are identical — same guard conditions, same two-line body. Either collapse them or extract a helper (consistent with how toggle_install_sdk_channel already abstracts the install manager side).

Minor

• Status messages use lowercase "release is stable, nightly is preview." but UI labels use title-case "Stable (default)" / "Nightly (preview)". Align the casing.
• Detail text says "On the Release row, Enter or Left/Right switches channel." — will mislead users who have Nightly selected. Should be "On the Channel row, ...".
• The Char('2') test block is testing pre-existing number-key-jump behavior — a short comment explaining what '2' does and why the assertion follows would help future readers.
• It's worth confirming the terminal height change from 32 to 36 rows in the install-failed render test reflects real additional content rather than masking an overflow.

[r0x0r]

All review comments have been addressed:

Critical: channel label always shows "Release"
Fixed — both install_sdk_choice_label and onboarding_menu_label now use "Channel:" as the neutral left-side label, so the row reads Channel: Stable (default) or Channel: Nightly (preview) consistently.

toggle_install_sdk_channel updates status unconditionally
Fixed — self.status = is now inside the if let block so the status only updates when the channel actually toggles.

Duplicate Left/Right arms in handle_onboarding_key
Fixed — collapsed into a single KeyCode::Left | KeyCode::Right arm.

Minor items:

• Casing aligned: detail text now says "Release is stable..." and "switches Release and Nightly" to match UI title-case.
• Detail text already said "On the Channel row, ..." in the pushed branch; confirmed correct.
• Added a comment to the Char('2') test block explaining that '2' jumps directly to the second actionable row (Channel) and activates it.
• Investigated the 32→36 height change: all assertions pass at height 32 as well, so the extra rows were unnecessary. Reverted to 32 to avoid masking overflow.

comments addressed