feat(desktop): enable fuzzy @-search with directory support in Composer

[SuMuxi66]

Summary

Restore the v1-style fuzzy @file search behavior in the Composer so that
an intermediate directory segment in a query (e.g. @planind) surfaces
nested matches such as src/planind/index.tsx, and directory names like
assets/ are selectable from the @-menu.

Previously the backend matched only on d.Name() (basename), the frontend
filter reduced searchEntries to basename-only, and SearchFileRefs
hardcoded IsDir: false for all results — so deep paths and directories
were invisible to the @-menu.

Closes #3769.

What changed

• internal/fileref/search.go

  • Search now returns []SearchResult (with Path and IsDir) instead of
    []string.
  • Matches are collected into three tiers: basename hits (preferred), path-
    segment hits (fallback), and directory-name hits (lowest). A dirQuota
    of 5 ensures directories are never fully crowded out by file matches.
  • pathSegmentContains(relSlash, queryLower) checks any slash-separated
    path segment for a match.
  • Noise directories (.git, node_modules, build, dist, ...) are
    still skipped. The public API signature is unchanged.

• desktop/app.go

  • SearchFileRefs now propagates r.IsDir from SearchResult to
    DirEntry.IsDir, fixing a pre-existing bug where all results were
    hardcoded IsDir: false.

• internal/cli/complete.go

  • Adapted to the new []SearchResult return type (CLI only uses Path).

• desktop/frontend/src/lib/atMatches.ts (new)

  • Pure helper filterAtMatches(entries, searchEntries, atFrag) that
    matches against the full slash-normalized relative path and de-duplicates
    by e.name with the local list taking precedence.

• desktop/frontend/src/__tests__/at-matches.test.ts (new)

  • 5 cases covering nested-path surfacing, basename match, de-duplication,
    local-before-fuzzy ordering, and the empty-fragment legacy behavior.

• desktop/frontend/src/components/Composer.tsx

  • The atMatches useMemo now delegates to filterAtMatches. The rest
    of the @-menu logic, the atDir !== "" guard, and the directory
    navigation UX are intentionally untouched.

• internal/fileref/search_test.go (new)

  • 6 cases covering path-segment match, basename match, directory match
    with IsDir=true, sort order between tiers, and noise-directory skip.

Test results

• go test ./internal/fileref -count=1 — 6/6 PASS
• pnpm exec tsx src/__tests__/at-matches.test.ts — 5/5 PASS
• go build ./... — clean
• Manual Wails dev test — directories and files both surface correctly