feat: v0.3.0 re-release - add dashboard & sessions, hide pi option

- Usage dashboard: KPIs, model split, daily bars, year heatmap, recent requests
- Sessions management: list/preview/archive/delete with streaming preview
- Double-click daily bar opens per-model breakdown modal
- Paginated recent requests (30/page)
- Hide pi option from nav (kept code, just removed from UI)
- Timezone fixes: today/day_key/heatmap now use local calendar day
- Streaming preview with 20MB byte cap + 500-char frontend clip
- README attribution to kimicode-dashboard (MIT, JochenYang)

src-tauri/Cargo.lock

@@ -1091,6 +1091,12 @@ dependencies = [
  "percent-encoding",
 ]
 
+[[package]]
+name = "fs_extra"
+version = "1.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "42703706b716c37f96a77aea830392ad231f44c9e9a67872fa5548707e11b11c"
+
 [[package]]
 name = "futures-channel"
 version = "0.3.32"
@@ -1957,7 +1963,9 @@ dependencies = [
  "anyhow",
  "chrono",
  "dirs 5.0.1",
+ "fs_extra",
  "indexmap 2.14.0",
+ "regex",
  "reqwest 0.12.28",
  "rusqlite",
  "serde",
@@ -1967,6 +1975,7 @@ dependencies = [
  "tauri-plugin-opener",
  "tauri-plugin-single-instance",
  "toml 0.8.23",
+ "walkdir",
 ]
 
 [[package]]