feat: restore Flashgrep with release-based binary downloads

[wgqqqqq]

Summary

Restore local workspace search and its settings, fetching Flashgrep binaries from a public binary-only GitHub release instead of committing executables. Desktop CI, local packaging, and the Windows installer include only the binary for their target architecture.

Type and Areas

Type: Feature restoration, bug fix, CI

Areas: Desktop/Tauri, Web UI, workspace search services, installer, packaging, development scripts

Motivation / Impact

• Download pinned Flashgrep v0.2.16 assets from wgqqqqq/flashgrep-binaries, verify SHA-256 and size, reuse valid caches, and atomically replace invalid caches. Source code remains outside that public repository.
• Restore local search settings and indexing controls. Remote workspaces explicitly reject Flashgrep operations before remote I/O and hide related controls; generated operation capabilities declare this limitation.
• Prepare Flashgrep automatically for development. Reuse Sherpa libraries across worktrees or download with curl to support SOCKS proxies, and keep Vite/HMR ports and desktop window URLs consistent through OPENBITFUN_DEV_PORT / OPENBITFUN_DEV_HMR_PORT.

Verification

Re-run after rebasing onto upstream main:

• node --test scripts/dev-startup.test.mjs scripts/prepare-flashgrep-resource.test.mjs scripts/desktop-tauri-build.test.mjs scripts/prepare-dsh-profile.test.mjs — 39 passed.
• pnpm --dir src/web-ui exec vitest run src/infrastructure/config/components/RuntimeSettingsPages.test.tsx src/app/global-search/providers/interactiveCapabilitySearchProvider.test.ts src/tools/file-explorer/search/useWorkspaceSearchIndex.test.ts — 31 passed.
• pnpm exec tsc --noEmit -p src/web-ui/tsconfig.json — passed.
• pnpm run capabilities:generate, pnpm run check:github-config, pnpm run check:core-boundaries, and git diff --check — passed.

Earlier focused verification on this change:

• Remote search service unit tests: 7 passed, including rejection without provider I/O; operation registry tests: 19 passed.
• cargo test -p openbitfun-desktop --no-default-features --lib appearance::development_frontend_tests with cached Sherpa libraries — 2 passed.
• Verified hashes and architecture headers for all six public release binaries. Ran real indexing and content hydration using the macOS ARM64 binary.
• Built and launched macOS ARM64 development desktop on port 1432; confirmed the configured window URL and frontend API traffic. User confirmed the UI works.

Reviewer Notes

Full platform packaging is left to CI. Remote-workspace rejection is covered by unit tests; no live SSH, remote-control, peer-device, or detached-dispatch end-to-end session was exercised. This does not add remote Flashgrep support.

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.