fix(web): compare and product page show invented values for unknown specs #1193

Follow-up to #1186 / PR #1192 (found in its review).

Problem

When a spec is unknown, some rows print a made-up value instead of nothing:

• Compare (apps/web/src/components/compare/CompareWorkspace.tsx):
  • cell_5g shows "4G LTE" when has_5g is unknown.
  • mm_speaker defaults to "stereo" (valDefaultStereo).
  • conn_usb_gps defaults to "USB Type-C".
  • avail_announced defaults to "Đang mở bán chính hãng" (valDefaultAvailable).
  • reg_cert defaults to "Đạt chuẩn hợp quy Bộ TT&TT Việt Nam" (valDefaultRegulatory).
• Product page (apps/web/src/lib/product-spec-rows.ts): the 5G row prints "No" when neither specifications.cellular.has_5g nor has_5g is known, and the commercial status row defaults to "Đang kinh doanh" (availableStatus).

This is user-facing misinformation: a buyer reads a fact the catalogue does not have.

Fix

An unknown value is empty and follows #1186's rules:

• the product page hides the row, and the group when no row is left;
• compare shows "-" only where another column has a value, and hides the row when every column is empty.

Acceptance

• No invented value for an unknown spec in compare or on the product page.
• Known values are unchanged (a stored has_5g: false still reads "No").
• Tests cover the unknown and known cases.