feat(engine): theme zip importer with safety validation (Card theme phase 7)

Implements Phase 7 of CARD_PLAN.md — the entry point that takes a
user-supplied theme zip archive, validates it end-to-end, and
atomically unpacks it into the per-platform user themes directory.

Public API:
  import_theme(zip_path) -> Result<ThemeId, ImportError>
    Resolves user_theme_dir() and unpacks into <user>/<id>/.
  import_theme_into(zip_path, target_root) -> Result<ThemeId, ImportError>
    Test-friendly variant that takes the destination explicitly so
    unit tests never touch the global OnceLock override.

Safety guarantees enforced:
- 20 MB hard cap on archive size (read from the central directory
  before any extraction).
- Zip-slip path traversal rejected via ZipFile::enclosed_name plus a
  Component::Normal-only belt-and-braces check.
- Manifest parsed via ron::de and validated via the existing
  ThemeManifest::validate (Phase 2) — surfaces named diagnostics for
  missing-of-52, unknown keys, duplicate keys, and meta errors.
- Every referenced face + back rasterised through rasterize_svg as a
  structural validity check before any bytes hit the destination.
- Atomic install: writes to <root>/.<id>.tmp/ then std::fs::rename
  into place, with a recursive copy + remove fallback for cross-
  device renames. Failed extraction wipes the staging dir; the user
  themes root is never touched on error.
- Id collision with an existing theme dir rejected up front.

7 new tests covering the happy path plus six failure modes (missing
manifest, missing face, oversized archive, zip-slip, missing-file,
id collision). Tests build zips in tempfile::TempDir so they never
touch the real user themes directory.

Workspace deps: zip 8.6 (default-features off + deflate only),
tempfile 3.27 (dev only).

cargo check --workspace --all-targets / clippy --workspace
--all-targets -- -D warnings clean. cargo test could not be run in
this turn because cc disappeared from the sandbox; tests compile
under cargo check --tests and will run on a normal toolchain.

Cargo.toml

@@ -50,6 +50,18 @@ tiny-skia = "0.12"
 # preserving Rust-style structures the importer can validate.
 ron = "0.12"
 
+# Importer-only: reads user-supplied theme zip archives, validates
+# their contents, and unpacks them into the user themes directory.
+# Default features are disabled to keep the dependency footprint small;
+# only `deflate` is needed because the importer rejects other
+# compression methods anyway (see Phase 7 spec).
+zip = { version = "8.6", default-features = false, features = ["deflate"] }
+
+# Importer-only test dependency: tests build zip archives in a
+# scratch directory so they don't pollute the real user themes path
+# on the developer's machine.
+tempfile = "3.27"
+
 axum               = "0.8"
 sqlx               = { version = "0.8", features = ["runtime-tokio-rustls", "sqlite", "macros", "migrate"] }
 jsonwebtoken       = { version = "10", default-features = false, features = ["rust_crypto"] }