feat: keep what was loaded between runs, and start where you left off (#2)

Starting lazysecrets again now shows what you had loaded at once, and puts you back where you were.

- **Cache file** `~/.cache/lazysecrets/cache.json` (0600, written atomically): the Sources lists, vaults/scopes, secrets' metadata, Describe results and each list's cursor. **Never values**, and never repos (they're on disk anyway). Loaded on start; lists older than 2 minutes reload in the background as before. Saved every 30s when changed, and on quit.
- **Housekeeping:** entries not loaded for 30 days are dropped, as is everything from accounts no longer signed in (every cache key starts with the account ID). The file has a format version; another version is ignored and replaced. `cache: false` turns it off; `--demo` never uses it.
- **Sources are cached too:** Azure lists subscriptions over the network, so without this the Sources pane would still wait.
- **Go to** searches everything cached straight away, and its background loading now also refreshes lists from an earlier run instead of skipping them.
- **Where you left off:** the state file remembers the selected source, vault or secret per folder (the `--dir` or working directory, 100 most recent folders), and the next start in that folder goes back there quietly (no error if it's gone). Starting on a link skips it.

Docs: `cache:` in config.md, a Cache section, the place in State, and the README Privacy section now mentions the cache file.

Tests: cache round trip with a backend too slow to answer (lists and cursor come from the file), no value in the file, file mode 0600, old and signed-out entries dropped, back where it was left, not for another folder or a link.

This touches `app.go` near PR #1's changes (Model fields, the Update switch), so whichever merges second needs a small conflict fix.

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-authored-by: Karl-Einar Bergstrøm <karl-einarb@outlook.com>
Reviewed-on: #2

internal/ui/app/links.go

@@ -43,6 +43,9 @@ type nav struct {
 
 	sourceRow string // Sources row to select
 	stage     navStage
+	// quiet gives up without a word: going back to where lazysecrets
+	// was left, which may be gone.
+	quiet bool
 }
 
 type resolvedMsg struct {
@@ -460,7 +463,9 @@ func (m *Model) addLinked(c provider.Container) {
 }
 
 func (m *Model) navFail(format string, a ...any) {
-	m.flash = flash{text: "open: " + fmt.Sprintf(format, a...), isErr: true}
+	if !m.nav.quiet {
+		m.flash = flash{text: "open: " + fmt.Sprintf(format, a...), isErr: true}
+	}
 	m.nav = nil
 }