Profile-post backfill + prune disposable first-run persona on import

Two bugs the v0.6.2 Discover story was going to expose:

1) Many named personas don't have a profile post yet, so the planned
   profile-post-driven Discover listing would show them as headless.
   Existing personas predate the Phase 2d profile-post primitive, and
   imported personas arrive with a display_name but no matching post.

2) Fresh install always generates a blank disposable persona before
   the user can pick fresh-vs-import. If the user picks import, that
   blank persona lingers forever — visible in the Personas list, a
   potential default, confusing.

Fixes:

Profile-post backfill (node.rs):
- New `Node::backfill_profile_posts_for_named_personas`: scans posting
  identities, skips unnamed or already-covered ones, and emits a
  signed profile post at the persona's own `created_at` timestamp. Run
  once from `Node::open_with_bind` after migrations. Idempotent via
  the new `Storage::has_profile_post_by_author` check. Chronology is
  preserved (post timestamp matches persona creation), so a later
  genuine profile update the user authors always wins the `> old_ts`
  monotonicity check on receivers.
- `Node::create_posting_identity(name)`: if `name` is non-empty, emits
  the profile post inline so new named personas are Discover-able the
  moment they're created. Uses the new `publish_profile_post_as`
  helper, which signs with the persona's own secret (not the default
  posting secret) and propagates via `update_neighbor_manifests_as`.

First-run persona marker + targeted prune (node.rs + storage.rs + import):
- `Node::open_with_bind`'s auto-gen block now also writes
  `first_run_auto_persona_id = <hex>` into the settings kv. This marks
  the specific disposable persona from the fresh-install flow; later
  prune logic uses this id, not a "any empty persona" rule, so manual
  empty personas are never touched.
- `Node::try_prune_first_run_auto_persona`: deletes the marked id iff
  all four gates pass — still exists, no display_name, no authored
  posts, no authored reactions/comments, and it's no longer the
  current default. Any one failure → clear the marker and keep the
  persona. New storage helpers `has_any_post_by_author`,
  `has_any_engagement_by_author` back the check.
- `set_profile` clears the marker when a non-empty name lands on the
  marked persona (user claimed it).
- `storage.delete_setting(key)` — new one-line helper.
- `import_as_personas_cmd` in tauri-app calls the prune after import
  completes; the cmd's return message reports "(cleared blank starter
  persona)" when it fires.
- New `get_first_run_auto_persona_id` Tauri command so the frontend
  can filter the blank persona out of the Personas list while it
  still exists.
- Frontend `loadPersonas` filters the marker id out of
  `personasCache` before rendering.

Tests: 124 / 124 core tests pass.

crates/core/src/storage.rs

@@ -1677,6 +1677,11 @@ impl Storage {
         Ok(())
     }
 
+    pub fn delete_setting(&self, key: &str) -> anyhow::Result<()> {
+        self.conn.execute("DELETE FROM settings WHERE key = ?1", params![key])?;
+        Ok(())
+    }
+
     // --- Seen engagement tracking ---
 
     /// Get the seen engagement counts for a post (react_count, comment_count).
@@ -2662,6 +2667,49 @@ impl Storage {
     }
 
     /// Find posts authored by us that were intended for a specific circle.
+    /// True if the given author has at least one `VisibilityIntent::Profile`
+    /// post stored. Used by the startup backfill to avoid re-emitting a
+    /// profile post for personas that already have one.
+    pub fn has_profile_post_by_author(&self, author: &NodeId) -> anyhow::Result<bool> {
+        let count: i64 = self.conn.query_row(
+            "SELECT COUNT(*) FROM posts
+             WHERE author = ?1 AND visibility_intent = '\"Profile\"'
+             LIMIT 1",
+            params![author.as_slice()],
+            |row| row.get(0),
+        ).unwrap_or(0);
+        Ok(count > 0)
+    }
+
+    /// True if `author` has any row in the posts table (any intent / visibility).
+    /// Used to decide whether the fresh-install auto-gen persona has been
+    /// "used" and should therefore NOT be pruned on import.
+    pub fn has_any_post_by_author(&self, author: &NodeId) -> anyhow::Result<bool> {
+        let count: i64 = self.conn.query_row(
+            "SELECT COUNT(*) FROM posts WHERE author = ?1 LIMIT 1",
+            params![author.as_slice()],
+            |row| row.get(0),
+        ).unwrap_or(0);
+        Ok(count > 0)
+    }
+
+    /// True if `author` has authored a reaction or a comment. Same purpose as
+    /// `has_any_post_by_author` but for the engagement tables.
+    pub fn has_any_engagement_by_author(&self, author: &NodeId) -> anyhow::Result<bool> {
+        let reacts: i64 = self.conn.query_row(
+            "SELECT COUNT(*) FROM reactions WHERE reactor = ?1 LIMIT 1",
+            params![author.as_slice()],
+            |row| row.get(0),
+        ).unwrap_or(0);
+        if reacts > 0 { return Ok(true); }
+        let comments: i64 = self.conn.query_row(
+            "SELECT COUNT(*) FROM comments WHERE author = ?1 LIMIT 1",
+            params![author.as_slice()],
+            |row| row.get(0),
+        ).unwrap_or(0);
+        Ok(comments > 0)
+    }
+
     pub fn find_posts_by_circle_intent(
         &self,
         circle_name: &str,