refactor: AI chat panel layout shell

[datlechin]

Summary

PR A of the chat panel redesign tracked in docs/refactor/ai-chat-redesign.md. Layout-only — no provider behavior change. Slot for B-H follow-ups.

• Inspector tab picker at the top of the right inspector. Pure SwiftUI Picker(.segmented) with Details and AI Chat. The window toolbar no longer carries any chat chrome.
• Slim empty state (sparkles icon, single title, caption). Drops the previous big illustration and starter prompt chips. Matches VS Code's chat panel weight.
• Single-row composer footer carries every chat action: @ mention, / slash commands, mode picker (Ask / Edit / Agent), model picker, history menu, new conversation, send/stop. Replaces the old in-content header.
• AIChatMode enum (ask / edit / agent) added to AISettings, persisted across launches. Mode picker writes to settings; does not yet gate provider behavior or tools (slated for PR D).
• @ObservationIgnored removed from MainContentCoordinator.rightPanelState so SwiftUI views inside NSHostingController re-render when the coordinator wires up the state on first display.
• Deletes MainWindowToolbar+AIChat.swift (no longer needed) and AIStarterPrompts.swift (chips removed).

Test plan

• Build the app, open a SQLite/Postgres connection.
• Toggle the inspector. The right pane shows Details | AI Chat segmented picker at the top.
• Switch to AI Chat. Empty state: sparkles icon, "Ask AI about your database", "AI responses may be inaccurate". No starter chips.
• Composer footer single row in order: @ ⌘ Ask▾ <model>▾ ... ⏱ ✏︎ ↑.
• Mode picker: pick Edit/Agent. Setting persists across app relaunch.
• Click ✏︎ (new conversation), ⏱ (history) — both work as before.
• Switch to Details tab. AI chat state is preserved when switching back.
• Disable AI in Settings. Inspector tab picker disappears, panel falls back to Details only.

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