Iteration 4b: save / save as / new / load with project switching

Adds the rest of the track-2 lifecycle commands (ADR-0015 §11)
and the project-switching machinery they need at runtime.

Temp vs named distinction: replaced the fragile naming heuristic
with an explicit `[temp]` marker in the directory pattern
(`<YYYYMMDD>-[temp]-<word>-<word>-<word>`). validate_user_name
already rejects brackets, so user-typed names can never collide
with a temp marker. The status bar shows `[TEMP] <Display Name>`
for temp projects; the prettifier strips both the date and the
marker so display names are clean.

save / save as: temp project's `save` opens a path-entry modal
(acts as save as); named project's `save` reports "already
auto-saved; use `save as`". `save as` always prompts. Relative
names resolve under <data-root>/projects/; absolute paths used
as-is. Copy excludes the per-process lock file; everything else
(.db, yaml, csvs, history.log) is copied.

new: closes current project, creates a fresh auto-named temp,
switches.

load: opens a picker. List sub-mode shows projects in the active
data root, sorted newest-first by project.yaml mtime; arrow keys
navigate, Enter loads, `b` switches to a path-entry sub-mode for
projects elsewhere, Esc cancels. Empty data root jumps straight
to path entry.

Runtime: `Session` holds Option<Project> + Option<Database> so
project switches can drop old (releasing lock + stopping worker)
before opening new -- required for the "load my own current
project" case. `perform_switch` handles Load / SaveAs / NewTemp
uniformly.

Tests: 332 passing (270 lib + 9 + 5 + 6 + 16 new + 9 + 17),
0 failing, 0 skipped. Clippy clean.

src/action.rs

@@ -36,4 +36,27 @@ pub enum Action {
     Rebuild {
         source: String,
     },
+    /// Open the load-picker modal. Runtime lists projects in
+    /// the active data root and posts back as
+    /// `AppEvent::LoadPickerReady`.
+    OpenLoadPicker,
+    /// Switch to the project at `path` (absolute or relative
+    /// to the active data root). Runtime drops the current
+    /// project, opens the new one, refreshes app state.
+    LoadProject {
+        path: std::path::PathBuf,
+        source: String,
+    },
+    /// Save the current project to `target` and switch to it.
+    /// `target` is a name or absolute path; relative names
+    /// resolve against `<data-root>/projects/` per ADR-0015 §1.
+    SaveAs {
+        target: String,
+        source: String,
+    },
+    /// Close the current project (auto-save guarantees state
+    /// is on disk) and create a fresh auto-named temp.
+    NewProject {
+        source: String,
+    },
 }