feat: multi-select picker for batch document attachment

[ericfitz]

Summary

Extend the document-editor-dialog (or a sibling flow) to support attaching multiple documents at once via a delegated content picker (Google Drive, Confluence, OneDrive, etc.).

Context

#626 ships single-file picker integration: the source selector in the document-editor-dialog lets the user pick one file from Google Drive, the dialog auto-fills name/URI from the picked file, and the user reviews per-document fields (description, include_in_report, timmy_enabled) before saving.

The Google Picker JS API and the planned Confluence/OneDrive providers all support multi-select natively. Users attaching several documents from the same source today have to open the dialog once per file.

Scope

• A multi-select flow that produces one document attachment per picked file in a single user gesture.
• A reasonable per-document editing experience: either a list-editor variant of the dialog (per-file name/description) or a streamlined "create now, edit later" path that drops the user into the documents list with the new rows highlighted for follow-up.
• Source-selector behavior: multi-select should only be offered for sources that natively support it; URL-paste remains single-file.

Out of scope

• The single-file picker flow shipped in feat: Google Drive picker integration for delegated document attachments #626 (remains the default).
• Multi-source picking in a single gesture (e.g. Drive + Confluence at once).

Blocks on

• feat: Google Drive picker integration for delegated document attachments #626 landing on dev/1.4.0.

Notes

• Worth re-evaluating once the Confluence (chore(deps-dev): bump @angular/cli from 20.3.12 to 21.0.2 #249 sub-project 2) and OneDrive (chore(deps-dev): bump @angular/cli from 20.3.12 to 21.0.2 #249 sub-project 3) providers ship — the multi-select UX should be designed once and applied to all delegated providers, not per-provider.
• Original single-file design rationale: most threat models have 0–3 attached documents, per-doc editing is meaningful, and single-file kept the feat: Google Drive picker integration for delegated document attachments #626 scope focused.