feat(android): accept shared files as Nextcloud uploads

[veryCrunchy]

User problem

Nextcloud Native does not appear as an Android Share target, so files from camera, gallery, document, and editor apps cannot be uploaded directly into a chosen Nextcloud folder.

Acceptance criteria

• Register narrowly scoped Android ACTION_SEND and ACTION_SEND_MULTIPLE entry points for supported content URIs.
• Open a native account and remote-folder destination picker before upload.
• Resolve display name, MIME type, and size defensively without trusting the sending application.
• Persist URI access when permitted or stage an app-private durable snapshot before temporary permission expires.
• Stream large uploads without loading complete files into memory.
• Use the existing durable transfer queue with progress, cancellation, retry, and process-death recovery.
• Preserve the original source; never mutate or delete content owned by the sending application.
• Handle unavailable URIs, revoked grants, unknown sizes, multiple files, duplicate names, quota failures, and authentication expiry with useful recovery states.
• Never expose account credentials, server endpoints, or private content in intent data, logs, diagnostics, or notifications.
• Add deterministic unit and Android integration coverage for single and multiple shares, large streams, permission expiry, cancellation, and activity recreation.

Relationships

• Parent: [EPIC] Files client and advanced sync #11
• Builds on transfer progress and cancellation in [SYNC-006] Transfer progress and cancellation #112 and resumable uploads in [SYNC-007] Resumable large uploads #113.
• Complements Android DocumentsProvider parity in [SYNC-016] Android DocumentsProvider parity #122.