Release Signboard 1.7.0

[cdevroe]

Summary

• prepare the focused Signboard 1.7.0 release from the 1.6.0 baseline
• include date rollover, updater, Obsidian metadata, due-ordering, Omarchy, Arch packaging, performance, CLI, MCP, and documentation improvements
• exclude the Smart Board Actions, cloud-provider, Normal/Advanced profile, and portable sharing work reserved for 2.0
• add native GitHub Actions builds for Linux x64 and ARM64 release packages

Validation

• 31 focused non-UI test scripts passed
• Playwright: 79 passed
• signed universal macOS packaging completed
• Windows universal installer packaging completed
• release artifact validation will run against the complete native Linux artifact matrix before publishing