fix(remote): stabilize workspace session restore

[bobleer]

Summary

• classify registered remote POSIX roots before native absolute-path validation so external command discovery degrades cleanly on Windows hosts
• single-flight startup reconnects by SSH connection and make matching live backend connects idempotent
• restore persisted desktop and CLI peer sessions before applying model updates
• add regression coverage for shared SSH profiles and restore-aware model synchronization

Type and Areas

Type: regression fix

Areas: desktop/Tauri, Rust SSH services, web UI, CLI peer host

Motivation / Impact

Remote workspace startup could issue concurrent connects for every workspace sharing one SSH profile. Each completed connect replaced and disconnected the previous handle, causing SFTP failures and remote session operations to fail. Remote POSIX paths were also rejected as non-absolute on Windows during external command discovery, and model updates could target a session that had not yet been restored into the runtime.

This change deduplicates reconnect work, preserves a matching live transport, classifies remote roots before host-native path validation, and restores session state before model mutation.

Verification

• pnpm --dir src/web-ui run test:run src/features/ssh-remote/SSHRemoteProvider.test.tsx src/flow_chat/services/flow-chat-manager/MessageModule.test.ts — 21 passed
• pnpm run type-check:web — passed
• cargo test -p bitfun-services-integrations --features remote-ssh,remote-ssh-concrete explicit_connect_reuses_a_matching_live_connection -- --nocapture — passed
• cargo check -p bitfun-desktop — passed
• cargo check -p bitfun-cli — passed
• pnpm run fmt:rs — passed
• git diff --check — passed

Reviewer Notes

AI-assisted: yes. Testing level: fully tested with focused frontend and Rust coverage plus both affected host compile checks.

No user-facing strings or locale resources changed.

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.