Add plan sharing via server API (#182, Option A)

[erikdarlingdata]

Summary

• Share button with two-phase consent dialog and user-configurable TTL (1 day to 1 year)
• One-chance delete button (token held in memory only, lost on page navigation)
• Server API (server/PlanShare/) deployed on Hetzner cpx11 (Ashburn, $6.99/mo)
• SQLite storage with per-plan expiry and hourly cleanup
• Shared plans load via ?share=<id> query parameter
• Rate limited (10 shares/min/IP), 10MB payload max

Files

• New: server/PlanShare/ — .NET 8 minimal API (SQLite, self-contained deploy)
• Modified: Index.razor — Share button, consent modals, delete, shared plan loading
• Modified: app.css — Modal, TTL picker, delete button styles
• Modified: index.html — JS helpers (getBaseUrl, getQueryParam, copyToClipboard)

Test plan

• Share a plan → consent phase 1 (TTL picker) → phase 2 (confirm) → link generated
• Share URL loads plan in new tab
• Delete button removes shared plan (verified 200→404)
• TTL expiry: expired plans return 404, cleanup removes from DB
• Rate limiting returns 429 after 10 rapid shares

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