feat: rebrand app metadata to Pinner S3 Server

[pcfreak30]

Update Sia app metadata to reflect the Pinner S3 Server branding and LumeWeb GitHub org. The `s3d` app ID is intentionally preserved to avoid breaking existing registrations.

Changes:

• `internal/backend/sia.go` — new `SiaAppMetadata()` helper, canonical source for `sdk.AppMetadata`
• `internal/backend/factory.go` — replaces inline struct with `SiaAppMetadata()`
• `internal/backend/manager.go` — same, unifies description with the canonical one
• `internal/onboarding/onboarding.go` — maps `SiaAppMetadata()` fields into `ConfigResponse`

---

This pull request rebrands the application's metadata to Pinner S3 Server and centralizes how that metadata is defined and consumed across the service.

Changes:

• Introduces a single canonical source, SiaAppMetadata(), that returns the app's registration metadata for the Sia storage SDK.
• Updates the app name from "S3d" / "s3-server-panel" to "Pinner S3 Server".
• Updates the service URL from the Siafoundation/s3d repository to the LumeWeb/s3-server repository.
• Replaces hard-coded metadata in the backend factory, account client, and onboarding config endpoint with calls to the shared SiaAppMetadata() function.

Functional impact:

• Ensures consistent app identity (name, description, logo, and service URL) everywhere the application registers itself with the Sia SDK or exposes its configuration to clients.
• Eliminates drift between the backend's SDK registration and the onboarding/config endpoint.

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[github-actions]

Docker image for testing

Pull the image for this PR:

docker pull ghcr.io/lumeweb/s3-server:sha-25b114f

Or use in docker-compose:

services:
  s3-server:
    image: ghcr.io/lumeweb/s3-server:sha-25b114f

The image is rebuilt on every push to this PR's branch.