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

docs/config.md

@@ -16,6 +16,7 @@ sourcesView: grouped   # or flat; ` toggles it
 collapseGroups: true   # start with the Sources pane's groups folded; enter or space unfolds one
 updateCheck: true      # look for a newer release once a day (U and lazysecrets update work either way)
 history: true          # count uses of items for Recent (R)
+cache: true            # keep what was loaded (never values) for the next start
 providers:
   azure:                                 # applies to every az account
     enabled: true                        # default
@@ -102,6 +103,25 @@ count as using it: its record keeps when it was last used and how often,
 for Recent (`R`). Records kept only for usage are dropped after 90 days
 unused, and beyond the 100 most recent. `history: false` stops counting.
 
+It also keeps where you left lazysecrets in each folder (the selected
+source, vault or secret), and goes back there the next time it starts in
+that folder, unless it was started on a link.
+
+## Cache
+
+What lazysecrets has loaded is kept in
+`~/.cache/lazysecrets/cache.json` (`$XDG_CACHE_HOME`), readable only by
+you: the Sources lists, vaults and scopes, and secrets' names and metadata
+(tags, dates, references), never values. On the next start they show at
+once, and anything older than two minutes reloads in the background when
+you go to it, or when Go to searches it. Revealing and copying always
+fetch the value live.
+
+Lists not loaded for 30 days are dropped, and so is everything from an
+account you're no longer signed in to. Repos aren't cached: their files
+are read from disk anyway. Deleting the file is always safe, and
+`cache: false` stops using it.
+
 ## Updates
 
 Once a day (`updateCheck`), in the background, lazysecrets asks the