Add visible loading/error states, budget tracking, calendar view, and proactive AI suggestions

[zinkh]

Summary

• Dashboard and Projects now show skeleton placeholders while fetching and a retry-able error banner instead of silently swallowing failed API calls into console.error (the "blank page on error" issue).
• Add a Budget Tracking section to the Dashboard comparing estimated project budgets against actual paid-invoice totals, per project and overall.
• Add a proactive AI Suggestions panel that surfaces overdue milestones, projects falling behind schedule, and overdue invoices, each with a clearly-labeled "Draft with AI" action that opens the agent chat with a prefilled (user-reviewable) draft message.
• Add a new Calendar page (month grid + this-week list) showing milestones and tasks across projects, wired into routing, sidebar nav, and i18n (FR/EN).

Test plan

• npm run lint (TypeScript type-check) passes
• npm run build succeeds
• Manual click-through of Dashboard, Projects, and Calendar in a real environment with live Supabase credentials (not available in the sandbox this PR was authored in — auth couldn't be exercised past /login)

🤖 Generated with Claude Code

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