v0.5.6 - iCal Subscriptions, MCP Server, and CNY Currency

[bscott]

Summary

• Add subscribable iCal URL that calendar apps can poll for live updates ([FR]: .ics URL in addition to file #83)
• Add MCP server binary for AI assistant integration (Claude Desktop, Claude Code)
• Add CNY (Chinese Yuan) currency support (New currency: CNY #95)
• Add configurable Base URL setting for reverse proxy deployments
• Centralize URL construction with X-Forwarded-Proto/X-Forwarded-Host support

Changes

iCal Calendar Subscription

• Token-protected /ical/:token public endpoint
• Toggle, copy URL, and regenerate token from Settings
• Subscribe button on Calendar page
• Subscription-friendly iCal properties for hourly refresh

MCP Server

• New cmd/mcp/main.go binary with 6 tools (list, get, create, update, delete, stats)
• Uses official Go MCP SDK v1.3.0 over stdio transport
• Reuses existing service layer, no new business logic
• Included in Dockerfile as subtrackr-mcp binary

Base URL

• New setting in Settings page for external URL override
• buildBaseURL() helper checks: configured URL > forwarded headers > request host
• Used by iCal URLs and password reset emails

Files Changed

• New: cmd/mcp/main.go, internal/handlers/url.go
• Modified: service/settings, handlers/subscription, handlers/settings, handlers/auth, middleware/auth, cmd/server/main, Dockerfile, README, templates, go.mod

Test Plan

• go build ./... compiles both binaries
• go vet ./... passes
• go test ./... all tests pass
• MCP server responds to tools/list and initialize over stdio
• Enable iCal subscription in Settings, copy URL, verify valid iCal content
• Disable subscription, verify /ical/:token returns 404
• Regenerate token, verify old URL stops working
• Set Base URL, verify iCal subscription URL reflects it
• Test behind reverse proxy with X-Forwarded headers

Closes #83
Closes #95