fix(browser): clarify empty network captures

[jun0315]

Description

Improves browser network behavior when a capture produces no visible entries.

This PR adds structured empty_reason and empty_hint fields so users and agents can distinguish between:

• no requests captured yet
• JS fallback capture being installed after requests already fired
• default filtering hiding all captured requests
• --filter, --failed, --since, or --until removing all entries

It also hardens the fresh capture path: if native network capture startup fails, browser network now attempts the JS interceptor fallback instead of failing before
fallback is tried.

Related issue: Partially addresses #1473

Type of Change

• Bug fix
• ✨ New feature
• New site adapter
• Documentation
• ♻️ Refactor
• CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Documentation (if adding/modifying an adapter)

• Added doc page under docs/adapters/ (if new adapter)
• Updated docs/adapters/index.md table (if new adapter)
• Updated sidebar in docs/.vitepress/config.mts (if new adapter)
• Updated README.md / README.zh-CN.md when command discoverability changed
• Used positional args for the command's primary subject unless a named flag is clearly better
• Normalized expected adapter failures to CliError subclasses instead of raw Error

Screenshots / Output

npx vitest run --project unit src/cli.test.ts
# 163 passed

npm run typecheck
# passed

git diff --check
# passed