fix(search): use filesystem search for non-Git folders

[wgqqqqq]

Summary

Fix content search in ordinary folders such as Downloads when Flashgrep is enabled. Check the local Git worktree/HEAD prerequisite before selecting indexed search, so ordinary folders and repositories without a first commit use the existing filesystem content search.

Type and Areas

Type: Bug fix

Areas: Desktop/Tauri search routing, workspace-search integration, regression tests

Motivation / Impact

Previously the file manager sent these folders to Flashgrep and displayed a protocol error requiring a Git worktree with a HEAD commit. The shared availability check now covers all three desktop content-search entry points and index actions. The probe reuses existing Git discovery on a blocking worker without starting a daemon or counting files.

Verification

• cargo test --locked -p openbitfun-services-integrations --no-default-features --features workspace-search --test workspace_search_contracts — 5 passed; 1 existing live-daemon test ignored.
• cargo test --locked -p openbitfun-desktop --lib api::search_api::tests — 3 passed.
• pnpm run fmt:rs and git diff --check — passed.
• Regression coverage: ordinary folders, unborn repositories, committed repositories, nested directories, linked worktrees, bare repositories, and actual filesystem content matches in ordinary/unborn folders.

Reviewer Notes

AI-assisted; testing level: focused automated tests. No manual desktop UI verification. Remote workspace, remote control, Peer Device Mode, and Detached Dispatch were not exercised end to end. Remote-path refusal still runs before the local probe; its refusal-message test passes. No persisted shapes or wire contracts change.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.