feat(feedback): add categories and UI element picker

[jal-co]

What

• add a required feedback category: Bug, Idea, UI issue, or Other
• when UI issue is selected, offer an optional interactive element picker
• include the category and picked-element context in the Discord feedback embed

Why

Triaging feedback needs a type, and UI reports need the exact element instead of a prose guess.

How

lib/element-picker.ts overlays a crosshair mode: hover highlights the element, click captures a compact descriptor (bounded CSS path, visible text snippet, viewport rect), and Escape cancels. The dialog goes non-modal and invisible while picking, then restores with a removable element chip. The server function validates the category against a fixed set and caps every element field before embedding.

Testing

• pnpm --filter web typecheck
• pnpm --filter web test (83 passed)
• pnpm --filter web build
• pnpm --filter web build-storybook
• git diff --check
• browser-verified: category select, picker hint, hidden dialog during picking, element capture chip with remove, and dialog restore

Screenshots

• Picked element chip: /tmp/feedback-element-picked.png