Improve i18n coverage for commands, modals, and widgets

[vincentredrock]

Summary

Follow-up to #740 / #781. After adding zh-Hant support, the shipped locale registry covers core UI chrome, but many user-visible TUI strings still remain hardcoded in English. This issue tracks expanding localization coverage for commands, modals, widgets, approval dialogs, and status picker UI.

This should improve all shipped locales (en, ja, zh-Hans, zh-Hant, pt-BR), not just Traditional Chinese.

Current Gaps

Representative areas that still render English under non-English locales:

• Composer hints, including slash menu and submit/queue hints
• Config modal labels, scopes, edit footer, and hints
• Status line picker title, actions, labels, and hints
• Pending input preview headers and removable hints
• Approval and sandbox elevation dialogs
• Sub-agent modal status labels and detail prefixes
• Common command output for /queue, /task, /trust, /restore, /config, /statusline, /lsp, and /logout
• Tool and agent card labels

Scope

• Move high-frequency hardcoded UI chrome into localization::MessageId
• Add translations for every shipped locale: en, ja, zh-Hans, zh-Hant, pt-BR
• Prioritize short, visible UI chrome before lower-frequency generated/exported content
• Keep command names, keybindings, paths, env vars, model IDs, and tool names unchanged where they are technical tokens

Non-goals

• Model output language
• System prompts or model prompts
• Tool output or shell command output
• User-provided content
• Provider protocol/internal keys
• Full localization of generated AGENTS.md or exported /share HTML in the first pass

Suggested Phasing

Phase 1: high-frequency TUI chrome

• Composer slash menu hints
• Composer submit/queue hints
• Config modal section/scope/edit labels and footer
• Status line picker title/actions/prompt
• Pending input preview section headers

Phase 2: approval and sandbox elevation

• Approval field labels (Type, About, Impact, Params)
• Risk/category labels
• Approval option labels and destructive confirmation footer
• Sandbox elevation title, option labels, and descriptions

Phase 3: common command output

• CommandResult::error prefix
• /queue, /task, /trust, /restore, /config, /statusline, /lsp, /logout

Phase 4: lower-frequency surfaces

• Tool family labels
• Agent lifecycle labels
• Fanout summaries
• Sub-agent modal detail prefixes
• Skills/hooks/session/memory command output

Acceptance Criteria

• New localized strings are represented by shared MessageId entries or equivalent locale-aware APIs
• Every shipped locale has translations for newly localized strings
• Existing localization completeness tests continue to pass
• Render smoke tests cover at least the surfaces changed in each PR
• cargo fmt --all -- --check passes
• cargo test --workspace --all-features passes
• cargo clippy --workspace --all-targets --all-features passes
• Manual LANG=zh_TW.UTF-8 cargo run --bin deepseek smoke testing shows reduced English chrome in the targeted surfaces

[vincentredrock]

Suggested labels: enhancement, localization. The current bug label appears to have been applied automatically; this is intended as an i18n coverage enhancement rather than a bug report.

[vincentredrock]

Quick status update: #781 adds zh-Hant as a shipped locale and is currently mergeable. Once that lands, follow-up i18n coverage work for this issue should include zh-Hant translations for every newly added MessageId, in addition to en, ja, zh-Hans, and pt-BR.

Also noting that #812 is related to this issue, but its current PR scope appears broader than the Phase 1 title/description: it changes 34 files with +1748/-469 lines and includes work beyond composer/config/status picker/pending input preview. It may be easier to review and land if the PR is either split by phase or its description is updated to reflect the actual scope.