feat: the right pane describes what's selected in the pane you're in (#9)

The right pane kept showing the selected secret when you moved to Vaults or Sources. Now it follows focus:

| focus | right pane |
|---|---|
| Sources | the account (group row), or the subscription / workspace / repo: name, note, ID / profile / path, host or remote, account, web page, vault count |
| Vaults | the vault / scope / file: name, note, URI / backend / format, source, web page, counts of loaded secrets, keys, certificates |
| Secrets | the secret, as before |
| the right pane itself | what it described when you went in; `shift+tab` goes back there |

All but the secret use the `Fields` pane (#7), so `j`/`k`, `/` and `y` work on them; `Detail` loses the error message it got in #4, which is now a red "problem" row in `Fields`. A value revealed from a left pane stays in view until hidden, so `space` from any pane still shows it. `tab` from a subscription goes to Vaults as before; only an account row skips to its details.

This also changes the startup view: focus starts in Sources, so the right pane describes the first subscription instead of the first secret (golden output updated). Tests that read the secret pane focus Secrets first; a new test walks secret → vault → subscription → into the pane → back, and reveals from Sources.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Karl-Einar Bergstrøm <karl-einarb@outlook.com>
Reviewed-on: #9

docs/keys.md

@@ -105,6 +105,13 @@ anything loads.
 
 ## Layout
 
+The pane on the right describes what's selected in the pane you're in: the
+account or subscription in Sources, the vault in Vaults, the secret in
+Secrets. Going into it (`4`, or `tab` from an account's row) keeps it on
+that; there `j`/`k`, `/` and `y` work on its rows, and `shift+tab` goes
+back. A value you reveal stays in view whichever pane you're in, until
+it's hidden.
+
 | Key | Action |
 |---|---|
 | `ctrl+←`/`ctrl+→` or `<`/`>` | move the divider between the left panes and the detail pane |