fix: address platform source review feedback

Tighten platform source persistence so legacy callers cannot silently relabel existing sessions, repair migration 24 when schema_versions drifts from the real schema, and polish the follow-up UI/error-handler review nits.

- only backfill platform_source when it is blank and raise on explicit source conflicts for an existing session
- make migration 24 verify both the sdk_sessions column and its index before treating it as applied
- expose platform_source from the functional session getters and add regression tests for source preservation and schema drift recovery
- add the required APPROVED OVERRIDE annotation for centralized HTTP error translation
- keep mobile source pills on a single horizontal row

src/services/sqlite/sessions/get.ts

@@ -17,7 +17,9 @@ import type {
  */
 export function getSessionById(db: Database, id: number): SessionBasic | null {
   const stmt = db.prepare(`
-    SELECT id, content_session_id, memory_session_id, project, user_prompt, custom_title
+    SELECT id, content_session_id, memory_session_id, project,
+           COALESCE(platform_source, 'claude') as platform_source,
+           user_prompt, custom_title
     FROM sdk_sessions
     WHERE id = ?
     LIMIT 1
@@ -38,7 +40,9 @@ export function getSdkSessionsBySessionIds(
 
   const placeholders = memorySessionIds.map(() => '?').join(',');
   const stmt = db.prepare(`
-    SELECT id, content_session_id, memory_session_id, project, user_prompt, custom_title,
+    SELECT id, content_session_id, memory_session_id, project,
+           COALESCE(platform_source, 'claude') as platform_source,
+           user_prompt, custom_title,
            started_at, started_at_epoch, completed_at, completed_at_epoch, status
     FROM sdk_sessions
     WHERE memory_session_id IN (${placeholders})