Turn repeated questions into a troubleshooting doc

Write a troubleshooting document from these repeated questions.

Structure by **symptom**, in the user's words:
1. Heading = what the user sees or types when asking. `"It won't save"`, not `"Persistence layer errors"`. *Users search with their symptom; they do not know the cause.*
2. Under each symptom, the checks in order of **how often that cause is the answer** — not in logical order.
3. Per check: what to do, what a good result looks like, what it means if not.
4. The resolution for each branch.
5. When to stop and escalate, and what to include when doing so.

Then:
- **Symptoms that map to more than one cause** — these need a distinguishing question up front.
- Questions that are actually a product problem, not a documentation problem. *If three people ask how to do something, the interface is unclear and a doc is a workaround.* Say which these are.
- What to put in the error message itself so the question never gets asked.

Rules:
- Use the words the asker used, including informal phrasing. Search matches their words, not yours.
- *Where I did not tell you the cause of a symptom, write "cause unknown — investigate" rather than inventing one.* A wrong troubleshooting step costs more than a missing one.
- No step that says "check the logs" without saying which log and what to look for.