feat: implement conversations page

[pikann]

Summary

Moves conversations out of the agent detail page and into their own project-level section, so a user can browse every conversation across all agents in one place instead of digging into each agent's tab.

• Added a top-level Conversations item to the project sidebar nav (between Agents and Automation).
• New /projects/$projectId/conversations layout route: a two-pane view with a list of all conversations for the project (sorted newest first, showing agent, status, iteration count, trigger, and created date) on the left and the selected conversation on the right via <Outlet />.
• New /conversations/ index route: redirects to the most recently created conversation, or shows an empty state if the project has none.
• Simplified /conversations/$conversationId: now renders ConversationView directly — the old back-button header and standalone realtime/query wiring moved up into the shared layout route.
• Removed the "Conversations" tab from the agent detail page (agents/$agentId/index.tsx), since conversations now live under the dedicated route instead of being scoped per-agent.
• Added nav.conversations and the conversationsPage.* translation keys to all 9 supported locales (en, es, fr, ja, ko, pt-BR, ru, vi, zh-CN).
• Regenerated routeTree.gen.ts for the new/changed routes.

Test plan

• pnpm lint / typecheck
• Manually verify: project sidebar shows Conversations nav item, list loads and links to a conversation, empty state shows when a project has no conversations, agent detail page no longer shows a Conversations tab

[pullfrog]

✅ No new issues found.

Reviewed changes — PR moves the conversation list from the agent-detail "Conversations" tab into a dedicated project-level route, adds sidebar navigation and localized empty/detail states, and nests the existing conversation-detail route under the new layout.

• Add project-level Conversations route with a list layout and agent-aware rows in apps/web/src/routes/_authenticated/projects/$projectId/conversations.tsx.
• Add index redirect to the most recent conversation in apps/web/src/routes/_authenticated/projects/$projectId/conversations/index.tsx.
• Nest conversation detail under the new layout in apps/web/src/routes/_authenticated/projects/$projectId/conversations/$conversationId.tsx.
• Remove the conversations tab from apps/web/src/routes/_authenticated/projects/$projectId/agents/$agentId/index.tsx and update its loader.
• Update navigation in apps/web/src/components/app-shell/app-sidebar.tsx and add translated labels across eight locales.
• Regenerate routeTree.gen.ts to reflect the nested route tree.

  ^{｜ View workflow run ｜ Using Kimi K2 (free via Pullfrog for OSS) ｜ 𝕏}

[pullfrog]

✅ No new issues found.

Reviewed changes — This new commit adds a preload guard to the index-route redirect so hovering the Conversations sidebar nav no longer silently navigates away from the current page.

• Guarded redirect on preload in apps/web/src/routes/_authenticated/projects/$projectId/conversations/index.tsx by returning early from the loader when TanStack Router is speculative preloading.
• No changes to the rest of the PR since the prior review.

  ^{｜ View workflow run ｜ Using Kimi K2 (free via Pullfrog for OSS) ｜ 𝕏}