feat(tauri-app): profile view, Avatar component, handle navigation

UI half of the profile feature. Mirrors the previous three commits
so the user can browse and edit profiles.

* `<Avatar did cid name size>` — reusable avatar component.
  Falls back to an initial-letter (or "?" when name is empty) circle
  when `cid` is null. Resolves the blob through the standard PDS
  fetch path so it works for any author whose PDS the client can
  reach.
* `<UserProfileView handle on_thread_click current_user_did>` —
  public profile page. Fetches `GET /api/profile/<handle>` on
  mount, renders the avatar / display name / bio / counts /
  posts. The "edit profile" button is gated on
  `current_user_did === profile.did` so a user browsing
  someone else's profile can't issue an unintended `setMyProfile`
  against their own DID.
* `PostCard` now renders an inline `<Avatar>` + clickable handle
  button that calls a new `on_handle_click` prop. The clickable
  area replaces the previous dead `<a href>` (Tauri webviews
  have no router).
* `App.svelte` adds an `openUserProfile(handle)` handler that
  sets `selectedHandle` + `view = "user"` and mounts
  `<UserProfileView>`.
* New Tauri commands `profile_get_record` / `profile_set` in
  `lib.rs` + matching client helpers `getMyProfile` /
  `setMyProfile` in `client.ts`. The set command sends camelCase
  field names; the PDS endpoint (previous commit) round-trips them
  through `#[serde(rename_all = "camelCase")]`.
* Empty-state UX for users with no profile yet (new account, or a
  third-party-PDS author whose profile the AppView hasn't indexed
  yet): both the current-user "profile" view and the public
  "user" view render a hint ("// no profile yet — click 'edit
  profile' to set one up." / "// no profile yet.") instead of a
  blank bio box.
* NavRail / NavRailHarness `View` union extended with "user"
  so the navigation prop type accepts the new view.

crates/tauri-app/src-tauri/src/lib.rs

@@ -720,7 +720,65 @@ pub fn run() {
             pick_and_upload_image,
             show_notification,
             open_external_url,
+            profile_get_record,
+            profile_set,
         ])
         .run(tauri::generate_context!())
         .expect("error while running maarcadetweet");
 }
+#[tauri::command]
+async fn profile_get_record(
+    state: tauri::State<'_, AppState>,
+) -> Result<Option<serde_json::Value>, String> {
+    let sess = state
+        .store
+        .load()
+        .ok_or_else(|| "not logged in".to_string())?;
+    state
+        .pds
+        .get_profile_record(&sess.did, &sess.access_jwt)
+        .await
+        .map_err(|e| e.to_string())
+}
+
+#[tauri::command]
+async fn profile_set(
+    state: tauri::State<'_, AppState>,
+    fields: serde_json::Value,
+) -> Result<(), String> {
+    let sess = state
+        .store
+        .load()
+        .ok_or_else(|| "not logged in".to_string())?;
+    let display_name = fields
+        .get("displayName")
+        .and_then(|v| v.as_str())
+        .map(str::to_string);
+    let description = fields
+        .get("description")
+        .and_then(|v| v.as_str())
+        .map(str::to_string);
+    let avatar_blob_cid = fields
+        .get("avatarBlobCid")
+        .and_then(|v| v.as_str())
+        .map(str::to_string);
+    let banner_blob_cid = fields
+        .get("bannerBlobCid")
+        .and_then(|v| v.as_str())
+        .map(str::to_string);
+    state
+        .pds
+        .set_profile(
+            &sess.did,
+            &serde_json::json!({
+                "displayName": display_name,
+                "description": description,
+                "avatarBlobCid": avatar_blob_cid,
+                "bannerBlobCid": banner_blob_cid,
+            }),
+            &sess.access_jwt,
+        )
+        .await
+        .map_err(|e| e.to_string())?;
+    Ok(())
+}