feat: add permission mode selector visibility preference

[wsp1911]

Summary

• Add a global preference for showing the Flow Chat permission mode selector.
• Add a hide action in the native permission selector and a recovery toggle in Tool Permissions settings.
• Keep the default visible preference out of persisted config files.

Fixes #

Type and Areas

Type: Feature / UI/UX

Areas: Rust core, Web UI, i18n

Motivation / Impact

Users who keep Full Access enabled can hide the prominent permission mode selector without changing the active permission policy. The selector can be restored from Tool Permissions settings.

Verification

• pnpm run fmt:rs
• cargo test -p bitfun-core app_flow_chat_permission_mode_control_defaults_to_visible_without_persisting_default -- --nocapture
• pnpm run type-check:web
• pnpm --dir src/web-ui run test:run src/flow_chat/components/ChatInputWorkspaceStrip.test.tsx
• pnpm run i18n:audit
• git diff --check

All commands passed. Interactive verification was not run.

Reviewer Notes

• app.flow_chat.show_permission_mode_control defaults to true.
• The default true value is omitted during serialization; only an explicit hidden state (false) is persisted.
• The preference controls visibility globally, including ACP sessions; ACP permission ownership remains unchanged.

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.