Improve user search and profile components

[mschilling]

Improvements from PR #286 review feedback

The following suggestions from Copilot code review are worth addressing in a future iteration:

1. Extract shared utility functions (DRY)

File: user-profile.component.ts, profile.component.ts

formatDisplayName() and formatMemberSince() are duplicated across ProfileComponent and UserProfileComponent. Extract these into a shared utility or service.

2. Case-insensitive search

File: user-search.service.ts

Search is currently case-sensitive (Firestore prefix search limitation). Consider storing a lowercase displayNameLower field on user documents and querying against that for case-insensitive matching.

3. Smarter back navigation

File: user-profile.component.ts

goBack() always navigates to /search. Consider using Location.back() with a fallback to /search when there's no navigation history (e.g. direct link).

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From Copilot review on PR #286