feat(btw): persist side threads with opt-in memory generation

[wsp1911]

Summary

Persist /btw side threads as structured child sessions and add an explicit, default-off memory-generation setting for them.

Fixes #

Type and Areas

Type:

Feature / regression fix / UI/UX / docs / test.

Areas:

Rust core, desktop/Tauri, web UI, remote workspace routing, memory settings.

Motivation / Impact

/btw conversations can now survive app restart and continue as normal persistent sessions. Child sessions retain their parent relationship and initial fork anchor, including in remote workspaces.

Memory generation from newly created BTW sessions remains disabled by default. Users can opt in through Memory settings; existing memory injection behavior is unchanged.

Verification

• pnpm --dir src/web-ui run test:run src/flow_chat/services/BtwThreadService.test.ts (7 passed)
• pnpm run type-check:web
• pnpm run i18n:audit
• pnpm run fmt:rs
• cargo test -p bitfun-core btw_session --lib -j 1
• git diff --check upstream/main...HEAD
• Manual: reopened an existing persisted remote session and verified /btw restores the parent coordinator session, keeps the child session visible, and starts execution.

Reviewer Notes

• New BTW sessions use durable Standard storage with relationship.kind = Btw; they are no longer transient sessions.
• The implementation is forward-only. Legacy BTW-shaped records are not migrated or reconstructed into structured child relationships.
• BTW memory eligibility is persisted at child-session creation, so changing the setting does not retroactively enroll existing BTW transcripts.
• btw_ask_stream and btw_cancel are now explicitly remote-routed.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, including the incomplete local Rust test.
• User-facing settings copy and locales are updated.