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

cmd/lazysecrets/main.go

@@ -334,6 +334,21 @@ Flags:
 		onActive = srv.Touch
 	}
 
+	// What was loaded last time, and where it was left in this folder.
+	var cacheFile, placeDir string
+	if !*demo {
+		if d, err := config.CacheDir(); err == nil && config.On(cfg.Cache) {
+			cacheFile = filepath.Join(d, "cache.json")
+		}
+		placeDir = *dir
+		if placeDir == "" {
+			placeDir, _ = os.Getwd()
+		}
+		if abs, err := filepath.Abs(placeDir); err == nil {
+			placeDir = abs
+		}
+	}
+
 	m := app.New(providers, app.Options{
 		Timeout:        *timeout,
 		Icons:          *icons,
@@ -353,11 +368,16 @@ Flags:
 		Commands:       cfg.CustomCommands,
 		Version:        versionString(),
 		Store:          st,
+		CacheFile:      cacheFile,
+		Dir:            placeDir,
 		Open:           open,
 		Styles:         &theme,
 		DefaultFg:      defaultFg,
 	})
 	p = tea.NewProgram(m)
-	_, err = p.Run()
+	final, err := p.Run()
+	if fm, ok := final.(app.Model); ok {
+		_ = fm.Persist() // a cache it couldn't save is no reason to fail
+	}
 	return err
 }