feat: add editable channel images

docs/TASKS/content/009-channel-setup-preview.md

@@ -15,6 +15,8 @@ Make channel setup feel like configuring real social destinations and make conte
 - Show configured channels as compact, network-shaped preview pages.
 - Show a banner/profile preview while creating a channel.
 - Use existing channel fields: name, network, handle, and external URL.
+- Let managers edit existing channel fields.
+- Let managers upload portrait and banner images for channels.
 - In the content creation editor, show a channel setup invitation when the workspace has no configured channels.
 
 ## Validation
@@ -22,6 +24,7 @@ Make channel setup feel like configuring real social destinations and make conte
 ```bash
 cd frontend
 npm run build
+dotnet build backend/Socialize.slnx
 ```
 
 ## Acceptance Criteria
@@ -30,3 +33,5 @@ npm run build
 - [x] The channels page content area shows preview cards for configured channels.
 - [x] The create-channel form includes a live network-style banner/profile preview.
 - [x] Content creation invites the user to configure channels first when none exist.
+- [x] Existing channels can be edited after creation.
+- [x] Channel portrait and banner images can be uploaded and shown in previews.