feat: Takeout walkthrough, design-system restyle, replacing import

Adds a seven-step in-app guide to exporting subscriptions from Google
Takeout, with the real URLs opened in the system browser.

Restyles the app onto the supplied design system: slate/sky palette,
9-15px type ladder, outline-first controls, tiered radii, borders for
separation and shadows only for elevation. Light and dark are both
designed, with a System/Light/Dark control and a pre-paint script so
the window does not flash light on a dark machine.

Importing now replaces the subscription list rather than merging, per
request. Because that can delete downloaded files, a confirmation
dialog names exactly what will go first.

src-tauri/src/commands.rs

@@ -3,7 +3,9 @@
 use crate::db::Db;
 use crate::downloader::{self, Progress};
 use crate::feed;
-use crate::models::{ChannelWithCount, DownloadState, FeedFilter, FeedItem, Prereqs};
+use crate::models::{
+    Channel, ChannelWithCount, DownloadState, FeedFilter, FeedItem, ImportPreview, Prereqs,
+};
 use crate::net;
 use crate::takeout;
 use crate::thumbs;
@@ -113,12 +115,8 @@ pub async fn check_prereqs(state: State<'_, AppState>) -> Result<Prereqs, String
     })
 }
 
-#[tauri::command]
-pub async fn import_takeout_csv(
-    path: String,
-    state: State<'_, AppState>,
-) -> Result<usize, String> {
-    let raw = tokio::fs::read(&path)
+async fn read_channels(path: &str) -> Result<Vec<Channel>, String> {
+    let raw = tokio::fs::read(path)
         .await
         .map_err(|e| format!("Cannot read {path}: {e}"))?;
     // Takeout exports are UTF-8, sometimes with a BOM.
@@ -129,8 +127,42 @@ pub async fn import_takeout_csv(
     if channels.is_empty() {
         return Err("No channels found in that file.".into());
     }
+    Ok(channels)
+}
+
+/// Reports what a replacing import would add and destroy, so the UI can name
+/// the consequences before the user commits to them.
+#[tauri::command]
+pub async fn preview_takeout_import(
+    path: String,
+    state: State<'_, AppState>,
+) -> Result<ImportPreview, String> {
+    let channels = read_channels(&path).await?;
+    state.db.lock().await.preview_replace(&channels)
+}
+
+/// The imported CSV becomes the entire subscription list. Channels that are no
+/// longer in it are removed along with their videos, download records, and the
+/// downloaded files themselves — leaving those on disk would orphan gigabytes
+/// the app can no longer show or delete.
+#[tauri::command]
+pub async fn import_takeout_csv(
+    path: String,
+    state: State<'_, AppState>,
+) -> Result<usize, String> {
+    let channels = read_channels(&path).await?;
+
+    let doomed = state
+        .db
+        .lock()
+        .await
+        .paths_dropped_by_replace(&channels)?;
+    for p in doomed {
+        let _ = tokio::fs::remove_file(&p).await;
+    }
+
     let mut db = state.db.lock().await;
-    db.upsert_channels(&channels)
+    db.replace_channels(&channels)
 }
 
 #[tauri::command]