feat: track founder beta acquisition source

[fly1d]

Summary

• require Founder Beta applicants to identify the public channel where they found Wordwise
• keep the Chinese and English forms aligned across planned distribution channels, excluding V2EX
• extend the validation funnel from channel reach through first use so each failed step can be diagnosed independently

Validation

• parsed both issue forms as YAML and verified unique IDs, required source fields, and unique options
• npm run check
• npm run test:smoke
• npm run test:site
• npm run desktop:check

Review focus

• confirm the source question does not request private data
• confirm the funnel rules prevent zero-reader channels from triggering product churn

[fly1d]

Reviewed the complete acquisition-measurement diff at 66cebbf. Both issue forms add the same required source field with unique channel options, omit V2EX as intended, and request no private text or identifiers. The validation guide now separates channel reach, product interest, qualified applications, and first use, preventing a zero-reader distribution failure from being misdiagnosed as a product defect. Both YAML files parse successfully with unique field IDs and options. Local typecheck, unit/API tests, production build, browser smoke tests, landing-page tests, Rust formatting, Clippy, tests, and Cargo check all passed. No blocking findings.