AI chat: retry failed responses, thinking level, delete confirmation, Markdown tables

Chat panel gaps found while using 0.15.1.

• No retry after a failed response. When a turn errors out (provider error, network, tool failure) the only option is to retype the prompt. Add a Retry action on the failed turn that resends the same message.
• No thinking/reasoning level control in chat. reasoningEffort exists on model profiles (src/app/ai/models/types.ts) but there is no way to pick it per conversation. Expose a thinking-level picker in the chat header next to the profile select.
• Delete-conversation confirmation is an inline strip. "Delete this conversation permanently? Cancel / Delete" renders inline under the header. Use the shared confirmation dialog (src/components/ui/dialog/) as the close-unsaved-document prompt does, per the AGENTS.md destructive-confirmation rule.
• Markdown tables are badly styled. Tables in responses render with heavy grey row backgrounds, cramped cells and poor contrast (especially in light theme). Restyle, and add a comprehensive ChatMarkdown Storybook story set covering tables, nested lists, code blocks, links, blockquotes and inline code so Markdown rendering has a visual baseline.