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

internal/ui/app/favourites_test.go

@@ -421,8 +421,9 @@ func TestStarIsPerAccount(t *testing.T) {
 	d.keys("n")
 	d.typeText("shared note")
 	d.keys("enter")
-	if n := strings.Count(d.screen(), "shared note"); n != 2 {
-		t.Errorf("note shown %d times, want 2:\n%s", n, d.screen())
+	// On both accounts' rows, and in the selected one's details.
+	if n := strings.Count(d.screen(), "shared note"); n != 3 {
+		t.Errorf("note shown %d times, want 3:\n%s", n, d.screen())
 	}
 	d.keys("F")
 	var labels []string