feat: search bar (posts/replies/people/opportunities) + People widget filter

[arturasmckwcz]

Context

Search + People widget for the Posts page rebuild, per the prototype proposal. Search needs a backend query — blocked on need4deed-org/be#930 (search endpoint + SDK types).

The People widget (filter the feed to one community member's contributions) is scoped in two phases per final-approval feedback (2026-09-01, Round 3 point 6 on #969) to avoid coupling it to replies existing yet:

• v1 (this pass): filter matches conversations where the selected person authored the post. Reuses GET /post with an author-filter param (e.g. authorId) — no dependency on be#927 (replies) or a new endpoint, so this can ship as soon as be#930's base search work lands.
• v2 (follow-up, once be#927 replies ship): extend the same filter to also match conversations where the person authored a reply, via a query-only addition to the existing filter (LEFT JOIN reply ... OR reply.author_id = :id, DISTINCT post.id — no schema change). A match always surfaces the whole post/conversation card, reusing the existing post-card component (feat: Post card — author/tags/opportunity chips, three-dot edit/delete menu with confirm dialog #963) — no new result type or UI needed for v2.

Scope

• Search input surfaces matches across post/reply text, people, and opportunities (via be#930)
• People widget: browse/select a community member to filter the feed — v1 to their posts, v2 (follow-up) to posts + replies

Acceptance criteria

• Searching finds matching posts (including matches inside replies) and highlights/links to people or opportunities mentioned in results
• Selecting a person in the People widget filters the feed to posts they authored (v1)
• Clearing the filter/search returns to the full feed
• (v2, follow-up issue once be#927 ships) People-widget filter also matches conversations where the person authored a reply

Blocked on: need4deed-org/be#930
Part of Epic: #969 (see Round 3 corrections, 2026-09-01, point 6)