refactor: Switch from localStorage to sessionStorage and clean up on navigation

[thejuggernaut01]

Problem

Migration configurations (migration_${jobId}) were stored in localStorage, meaning they persisted across browser sessions indefinitely — even after the tab or browser was closed. This is unnecessary since migration configs are only needed during the active session. Additionally, draft connection strings (db_mover_draft_${dbType}) and migration configs were never cleaned up when users navigated away from the page, leaving stale sensitive data in storage.

Changes

client/src/pages/ConfigPage.tsx

• Switched localStorage.setItem → sessionStorage.setItem for storing migration configs (migration_${jobId}), so data is automatically cleared when the tab is closed.
• Added a useEffect cleanup that removes db_mover_draft_${dbType} from session storage when the user navigates away from the config page. This handles the case where a user enters connection strings (triggering draft auto-save) but navigates elsewhere without starting a migration or manually clearing the draft.

client/src/pages/MigrationPage.tsx

• Switched localStorage.getItem / localStorage.setItem → sessionStorage for reading and storing migration configs during retry.
• Added session storage cleanup to the existing SSE useEffect cleanup function. When a user navigates away from the migration page:
  • Reads the dbType from the stored migration_${jobId} config
  • Removes the corresponding db_mover_draft_${dbType} key
  • Removes the migration_${jobId} key itself

Why sessionStorage over localStorage

• Automatic browser cleanup: Session storage is cleared when the tab closes, so users don't need to worry about stale connection strings lingering.
• Scoped to the session: Migration configs are only relevant for the current session — there's no need to persist them across browser restarts.
• Combined with navigation cleanup: The useEffect cleanups handle the remaining case where data persists during in-app navigation within the same tab.

[Copilot]

Pull request overview

This PR updates the client-side migration flow to store migration configuration and draft connection strings in sessionStorage (instead of localStorage) and adds cleanup on route transitions to reduce persistence of sensitive data.

Changes:

• Switch migration config persistence (migration_${jobId}) from localStorage to sessionStorage for start + retry flows.
• Add navigation/unmount cleanup to remove db_mover_draft_${dbType} (Config page) and both draft + migration config (Migration page).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
client/src/pages/ConfigPage.tsx	Writes migration config to sessionStorage and removes draft data on unmount/navigation.
client/src/pages/MigrationPage.tsx	Reads/writes retry config via sessionStorage and removes draft + migration config on unmount/navigation.

Comments suppressed due to low confidence (1)

client/src/pages/ConfigPage.tsx:44

• sessionStorage.setItem() can throw (quota exceeded / storage blocked). Because this is inside the same try as the API call, a storage write failure would route into the catch path and report “Migration failed” even though the migration job may already have been started successfully. Consider guarding the storage write with its own try/catch and treating failures as a non-fatal loss of retry capability (e.g. log + toast warning, but still navigate).

      // Store migration config for retry functionality
      sessionStorage.setItem(
        `migration_${jobId}`,
        JSON.stringify({
          type: "copy",
          sourceUri: config.sourceUri,
          targetUri: config.targetUri,
          dbType: dbType,
        }),
      );

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