fix(ui): give feedback when the project name slugifies to empty

[CatJuly]

Summary

Clicking + to create a project silently does nothing when the typed name contains no ASCII word characters — for example a fully Chinese name like 知识工程.

Repro: Workspace top bar → + → type 知识工程 → press Enter or click +. Nothing happens: no project, no error, no visual change.

Root cause

handleCreate in ProjectSelector.tsx slugifies the input before submitting:

const id = newName.trim().toLowerCase().replace(/[^a-z0-9_-]+/g, '-').replace(/^-|-$/g, '')
if (!id) return   // ← fully non-ASCII names strip to "" and the click is swallowed here

The slug restriction itself is correct — the backend rejects anything outside [a-z0-9_-] (is_safe_project_id). The bug is purely that the constraint is invisible: the click is dropped with no feedback, which reads as a dead button.

Fix

Derive the slug live while typing (same regex, extracted to toProjectSlug). When the name is non-empty but the slug is empty:

• the input gets a red outline, aria-invalid, and a tooltip
• a small hint appears anchored below the input (absolutely positioned, so the top bar layout does not shift): "Use letters, numbers, hyphens or underscores" / "仅支持字母、数字、连字符和下划线"
• the submit button is disabled

Valid input behaves exactly as before — My App still creates my-app, plain ASCII names are unaffected.

Testing

• npm run typecheck — clean
• npm run build — clean; rebuilt frontend_dist bundle included
• Manual: Chinese-only input shows the invalid state and cannot submit; My App creates my-app; demo-2 creates unchanged; Escape still closes the form

Scope

Frontend only (ProjectSelector.tsx, two i18n keys, rebuilt dist). No backend or validation-rule changes.