docs: document dashboard WebSocket troubleshooting

[douglasmonsky]

Summary

• What changed: added a bounded troubleshooting sequence for an offline LiveView badge.
• Why it changed: headless captures can lose the dashboard WebSocket even when Symphony and its JSON API are healthy.
• User/developer impact: operators can distinguish a capture limitation from an actual service failure and know when to inspect the service log and an interactive browser.

Closes #424

Verification

• Focused tests: npx markdownlint-cli2 docs/symphony/operations.md — passed, 0 errors.
• Agent Maintainer profile(s): scripts/verify — full profile passed, run 20260724T192808210147Z-full-d7f0b4678694 (185 seconds; 30 passed checks, 2 non-failing warnings, 0 failures).
• GitHub checks: all 7 checks on commit 5d1af53 passed.
• Not run: none.

Contributor Checks

• I read CONTRIBUTING.md and CODE_OF_CONDUCT.md.
• This change contains no credentials, private data, or identifying records.
• User-facing behavior is reflected in documentation; no code or changelog behavior changed.
• Security-sensitive details were reported privately rather than placed in this pull request.

Cohesive-Change Override

• Override requested: no
• Why this is one cohesive unit: one documentation troubleshooting addition.
• Why smaller PRs would make the repository less coherent: not applicable.
• Tests/verification proving behavior is unchanged: focused Markdown lint, full local verification, and all GitHub checks passed.
• Behavior change: documentation only.