Support local favicon discovery for webapp install

[shbernal]

Summary

• Allow omarchy webapp install <name> <url> to run non-interactively without an icon argument.
• Try the web app's advertised favicon first, including relative icon paths, before falling back to Google's favicon service.
• Keep explicit icon URLs and saved local icon names working as before.

Tests

• bash test/omarchy-webapp-install-test.sh
• bash test/omarchy-cli-test.sh
• git diff --check upstream/dev...HEAD

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR refactors omarchy-webapp-install to discover a web app's declared favicon (via <link rel="icon">) before falling back to Google's favicon service, makes the icon argument optional in the non-interactive form, and adds a shell-based test that exercises the new behavior with a stubbed curl.

Changes:

• Adds helpers (normalize_url, url_origin, url_directory, absolute_icon_url, discover_icon_url, download_icon, use_default_icon) and makes icon optional in the CLI form.
• Fails with a clear error if no icon can be discovered/downloaded when none is provided.
• Adds a new test script covering local favicon discovery, Google fallback, saved local icon names, and missing-URL failure.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated 1 comment.

File	Description
bin/omarchy-webapp-install	Extracts URL/icon helpers, auto-discovers site favicon, falls back to Google, and updates arg parsing/usage doc.
test/omarchy-webapp-install-test.sh	New shell tests that stub curl and validate desktop/icon outputs across scenarios.

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