Decisions tab: report an unusable _Zone: header line once, as a file-level problem #1838

Problem

The decisions file may declare its zone once in its header, as _Zone: <zone>_ (#1836, PR #1837). When that line can't be used, nothing tells the owner or the hub:

• A line that doesn't parse (e.g. _Zone: Pacific Time_, two words) counts as no zone line at all. Every bare-time stamp in the file is quietly shown as written.
• A duplicated line (two _Zone: lines in the header) also counts as no zone. That's deliberate, since the parser never guesses between two zones, but it is silent too.
• An unknown zone (e.g. _Zone: XYZ_, or the template's <zone> left unfilled) warns on every bare-time card ("Not converted: the time zone is not one the tab recognizes"), with nothing pointing at the header line that caused it.

In all three cases the problem is one line in the header, but it either shows nowhere or shows on every card.

Fix

Report it once, as a file-level problem:

• Add DecisionsFileProblem value(s) for a _Zone: header line that doesn't parse, is duplicated, or names a zone resolveStamp doesn't know. The parser already reports open-missing / open-multiple / ruled-multiple the same way.
• Show it once at the top of the Decisions tab, naming the line and what to write (_Zone: PT_, _Zone: Europe/Berlin_, …).
• Decide whether the per-card unknown-zone flag stays for bare-time cards when the file problem already names the cause. Recommendation: drop it for stamps whose zone came from the file, and keep it for zones written in the stamp itself.

Testing

• Parser unit tests for each case (unparseable, duplicated, unknown, the unfilled <zone> template).
• A DecisionsView test that the file-level notice renders once.
• Add the case to the Playwright Decisions suite.

Origin: integration review of PR #1837.