feat(agent): improve chat workflows, navigation, and update UX

[yxmnshmily]

Summary

This draft contributes a coherent upstream-safe Agent/chat UX batch extracted from the local editor work. It keeps OpenChatCut branding and excludes private fork data, launcher code, and unrelated product-specific features.

Changes

• Add chat-local Cmd/Ctrl+A selection and direction-sensitive fast scroll navigation.
• Preserve native visible-text copy instead of triggering editor clip-copy shortcuts.
• Route editor shortcuts by focused surface so timeline actions do not fire in the media pool, Agent chat/input, or Inspector.
• Drag Finder files into the composer through the existing media import pipeline, with placeholder-first attachment and failed-reference cleanup.
• Drag one or multiple selected media-pool assets into the composer as structured references.
• Drag templates and other library resources into the composer without modifying the timeline.
• Localize template reference chips without mutating reference identity.
• Activate the 8 built-in creative workflows and fill concise locale-aware prompts, including custom workflows.
• Keep composer popovers inside the Agent workspace with resize, scroll, observer, Escape, and outside-dismiss handling.
• Enforce the official OpenChatCut product identity in API and Codex Agent runtimes.
• Compare the installed app version with official GitHub releases and add update notices/settings checks.
• Localize reusable design-style preset metadata while preserving saved style data.

Validation

• Full npm test: PASS.
• Focused structured-drag, attachment lifecycle, shortcut-scope, chat, workflow, popover, update, Settings, and design-style checks: PASS.
• npm run build: PASS (existing chunk-size/dynamic-import warnings only).
• oxlint on the added/modified Agent-drop files: no errors; pre-existing Timeline hook warnings remain.
• git diff --check: PASS.
• Delegated Open Code Review completed; no High/Critical findings remain.
• Sensitive/private scan: no TravelChatCut identity, local user paths, tourism demo data, keys, or tokens.

Integration notes

• Media-pool progressive import and batch placement are also extended by PR feat(media): complete media pool and resource library workflows #25. After feat(media): complete media pool and resource library workflows #25 lands, this draft should be rebased and overlapping lifecycle glue dropped where upstream already provides it.
• Timeline-specific context actions and preview behavior remain in PR Improve timeline, preview, and mixed caption workflows #23; this PR only contributes the shortcut surface marker needed by the routing contract.
• This remains Draft for maintainer review and conflict-minimizing rebase after the prerequisite batches land.

Scope intentionally excluded

• Private About dialog, company details, launcher scripts, TravelChatCut branding, demo content, credentials, and local runtime data.
• Product-specific desktop IPC and private update/install flows.
• Fork-only test-manifest and packaging topology.