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/notes_test.go

@@ -42,7 +42,7 @@ func TestNotesOnRows(t *testing.T) {
 	d.note("main prod")
 
 	screen := d.screen()
-	for _, want := range []string{"stripe live", "app secrets", "main prod", "note       stripe live"} {
+	for _, want := range []string{"stripe live", "app secrets", "main prod", "note       main prod"} {
 		if !strings.Contains(screen, want) {
 			t.Errorf("missing %q:\n%s", want, screen)
 		}