fix: discover npm components declared in webui-press config

[mohamedmansour]

Closes #370.

Problem

External components declared as npm packages in a webui-press config.json components array (e.g. "@mai-ui") were never discovered — the build failed with either Component path not found: /<project>/@mai-ui or Could not find node_modules/ directory (searched upward from /var/folders/...).

Two coupled causes:

1. webui-press mangled the specifier. Every configured source was resolved with current_dir().join(source), turning the npm specifier @mai-ui into an absolute path /<project>/@mai-ui. webui-discovery then classified it as a local path and failed.
2. Discovery anchored at the wrong directory. npm discovery walks up for node_modules/ from the build app directory. webui-press renders each page from a synthesized scratch dir in the system temp folder, which has no node_modules ancestor — so even a correctly-bare @scope could not be resolved.

Changes

webui-discovery

• Add find_node_modules_with_fallback(primary, fallback), used by resolve(): walk up from the app directory first, then from the process working directory (the project root the command was invoked in). The primary error is preserved when both fail. Normal webui build/serve is unaffected — the app dir is already inside the project, so the fallback never triggers there.
• Expose pub fn is_local_source(&str) -> bool as the single source of truth for path-vs-npm classification; classify_source now delegates to it.

webui-press

• Resolve configured component sources through webui_discovery::is_local_source: npm names/scopes stay bare (so discovery resolves them from node_modules), while local paths are still made absolute against the project root. Adds microsoft-webui-discovery as a direct dependency.

Why this approach

Discovery already receives the app directory; the only gap is that webui-press's app dir is a throwaway scratch dir outside the project. A cwd fallback fixes that at the source without a new BuildOptions field and without relocating scratch into the project tree (which would otherwise need to be git-ignored and watcher-ignored).

Testing

• cargo test -p microsoft-webui-discovery (34 passed) and -p microsoft-webui-press (45 passed), including new tests for the fallback resolver, is_local_source, and webui-press config source resolution.
• cargo fmt + cargo clippy clean on both crates.
• End-to-end: a press site with "@mai-ui" in config now builds and renders all 15 <mai-*> tags (button, link, text-input, spinner, divider, dropdown, listbox, menu-button, menu-item, menu-list, option, tab, tablist, tree, tree-item).

Compatibility

Behavior-preserving for existing builds. The fallback only adds a resolution path when the primary walk-up finds no node_modules; it never overrides a successful primary match. is_local_source is purely a refactor of existing classification logic.