feat: personal canvas dashboards for Rill Cloud

[nishantmonu51]

Lets project members (including viewers) create owner-only canvas dashboards in Rill Cloud, stored as virtual files in the admin DB. Gated by the new personal_canvases feature flag and the new create_personal_canvases project permission.

Backend

• New generic admin RPCs Create/Edit/Delete/Copy/Get/ListPersonalVirtualFile with a PersonalVirtualFileType enum (canvas in phase 1; extends to reports/alerts without new RPCs)
• Per-type lookup table in admin/server/personal_virtual_files.go drives path layout, YAML validation, and required permission
• FindVirtualFilesByPrefix for owner-scoped listing
• Migration 0094 grants create_personal_canvases to roles with read_prod
• builtInCanvasSecurityRule enforces owner-only access on admin-managed canvases (deny-by-default for non-owners)
• New CanvasSpec.annotations field carries admin_owner_user_id and admin_managed through parse/reconcile
• New runtime AI tool create_personal_canvas so the agent can build a canvas for the current user in Rill Developer

Frontend

• VirtualFilePersistence extends FileArtifact, overriding the new putBlob / fetchBlob hooks to route through the admin RPCs
• PersonalCanvasWorkspace reuses CanvasBuilder, CanvasEditor, and VisualCanvasEditing from web-common for parity with Rill Developer's visual editor (add widget, resize, inspector panel, code/visual toggle, save-as-default)
• WorkspaceHeader gains a showBreadcrumbs prop so cloud hosts can hide the file-tree breadcrumbs that only make sense locally
• New project home section lists the caller's personal canvases with a "create from blank or copy" dialog
• "Done editing" flushes pending edits, invalidates the canvas resource cache, and returns to the preview

INSERT DESCRIPTION HERE

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!