desktops: suppress dpkg conffile prompt on install (non-interactive)

[igorpecovnik]

Summary

On a system that previously imported a chromium conffile (e.g. `/etc/chromium/master_preferences`) from the package, `module_desktops install` blocks at:

```
Configuration file '/etc/chromium/master_preferences'
==> File on system created by you or by a script.
==> File also in package provided by package maintainer.
What would you like to do about it? ...
```

The install path was already passing `--force-confold`, but that alone doesn't suppress the prompt when both the user's file and the package's new version differ from the original — exactly our case with the browser-branding overlay in #854 (overlay writes `master_preferences`, chromium .deb also ships an updated version).

Fix

• Add `--force-confdef` alongside `--force-confold` on every `pkg_install` call. Combined semantic: "always take the default action (= keep local) on any conffile dpkg can't auto-merge, silently".
• Export `DEBIAN_FRONTEND=noninteractive` at the top of the install branch so apt itself doesn't open a TUI for debconf questions either.

6 `pkg_install` call sites updated — desktop packages, display manager, `armbian-plymouth-theme`, and the upgrade-tier path (`_module_desktops_change_tier`).

Test plan

• On a system where chromium was previously installed and `master_preferences` was imported from the package: re-run `armbian-config --api module_desktops install de=xfce tier=full` and confirm it runs to completion without a conffile prompt
• `/etc/chromium/master_preferences` remains the local version (whatever was on disk) after install
• Regression check: on a pristine system, fresh desktop install still works (conffile flags are no-op when there's nothing local to keep)

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 53eb3423-1211-4a29-86d1-eb21c72ee201

📥 Commits

Reviewing files that changed from the base of the PR and between a9bad9a and 7d212cf.

📒 Files selected for processing (1)

• tools/modules/desktops/module_desktops.sh

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch install-suppress-conffile-prompt

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