Iteration 5: export / import commands

Implements the `export` and `import` app-level commands per
ADR-0015 §11 + ADR-0007 amendment 1.

- `export [<path>]` writes a zip of project.yaml + data/ to
  <data-root>/YYYYMMDD-<projectname>-export-NN.zip by default,
  preserving the project's directory name as the single
  top-level folder inside the archive.
- `import <zip> [as <target>]` extracts an exported zip into
  a new named project and switches to it. Target name is
  derived from the zip's top-level folder by default; on
  collision the destination auto-suffixes -02, -03, ... up
  to -99 instead of refusing (deviates from §2's refuse-on-
  collision rule for save/save as; recorded as an amendment
  to ADR-0015 §11).
- Excludes playground.db and history.log from the zip.
- Path-traversal protection via zip::enclosed_name + post-
  resolution check that the extraction path stays inside
  the target directory.

Adds the zip = "5" dep with default-features = false +
features = ["deflate"] to keep the binary-size cost modest.

Test baseline: 370 passing, 0 failing, 0 skipped.

src/event.rs

@@ -77,9 +77,9 @@ pub enum AppEvent {
     LoadPickerReady {
         entries: Vec<crate::app::LoadPickerEntry>,
     },
-    /// A project switch (load / new / save-as) succeeded.
-    /// Carries the new display name + temp flag so App can
-    /// update the status bar.
+    /// A project switch (load / new / save-as / import)
+    /// succeeded. Carries the new display name + temp flag
+    /// so App can update the status bar.
     ProjectSwitched {
         display_name: String,
         is_temp: bool,
@@ -90,4 +90,14 @@ pub enum AppEvent {
     ProjectSwitchFailed {
         error: String,
     },
+    /// Export wrote a zip successfully. Carries the resolved
+    /// final path so the user gets a "wrote to: …" note.
+    ExportSucceeded {
+        path: std::path::PathBuf,
+    },
+    /// Export failed in a non-fatal way (target exists, IO
+    /// error, sequence range exhausted, …).
+    ExportFailed {
+        error: String,
+    },
 }