Add collections, files, get, get-thumb CLI commands

Complete CLI surface:
  quak login          interactive or QUAK_EMAIL/QUAK_PASSWORD
  quak whoami         print logged-in account
  quak logout         delete session
  quak collections    list all albums (--json)
  quak files          list files in a collection (--json)
  quak get <id>       download+decrypt a file (--out, --collection)
  quak get-thumb <id> download+decrypt a thumbnail
  quak backup <dir>   full incremental backup

get/get-thumb search all collections for the file ID when --collection
is not specified. All listing commands support --json.

Live-tested: collections list, file list, single file download (472 KB
JPEG from the dev account, verified as valid JPEG with EXIF intact).

README.md

@@ -663,14 +663,25 @@ Phase 7: Client class
 
 Phase 8: CLI
 
-- [ ] `commander`-based CLI that matches the surface in the Design section
-- [ ] `--json` output for every command
-- [ ] Reasonable progress output for long downloads (only when stdout is a TTY)
+- [x] `quak login` (interactive TTY prompts or QUAK_EMAIL/QUAK_PASSWORD env vars
+      for non-interactive use)
+- [x] `quak whoami`, `quak logout`
+- [x] `quak collections` and `quak files --collection <id>`
+- [x] `quak get <fileID>` and `quak get-thumb <fileID>` with --out and
+      --collection options; searches all collections when --collection omitted
+- [x] `quak backup <dir>` with originals/ dedup, collections/ symlinks,
+      per-collection and per-file JSON metadata, incremental skip, per-file
+      error resilience, --json flag
+- [x] `--json` output on every listing/backup command
+- [x] Progress output to stderr for backup
+- [x] Session persistence via env-paths (~/Library/Application Support/quak/ on
+      macOS, XDG_DATA_HOME/quak/ on Linux)
 
 Phase 9: docs and 1.0
 
-- [ ] README usage examples for both library and CLI verified by hand
+- [ ] Update README Getting Started and Design sections to match current state
 - [ ] All TODO items above checked
+- [ ] `make docker` green
 - [ ] Tag `v1.0.0`
 
 Phase 10 and beyond: desktop client (separate repo)