Frontend slice: app-tree consolidation, new project panels, account deletion UI

@hoogvliets isolated the frontend changes from backend and database work into a standalone AGPL disclosure slice. The structural core is an app-tree consolidation; the surface additions include project panels, an account-deletion banner, and manifest context.

The structural change moves shared providers, auth context, user profile context, the Supabase client, and utilities under a single frontend/src/app/ root and rewrites imports to follow. Two new pieces of app-level plumbing land with it: a ManifestsContext and a useProjectHandlers hook that centralizes project state and the operations on it.

On top of that, the project page is reorganised into separate document, explorer, and workflow panels with a shared divider component. The account layout picks up a model settings section and an account-deletion banner that pairs with the lifecycle changes in the backend PR. The diff also touches the assistant chat (model toggle, side panel), workflow modal and detail views, DOCX rendering, modals and row actions, and the tabular review and generation flows.

Package manifests and lockfiles are updated to keep Next.js, React, OpenNext, Supabase, and the markdown/DOCX/UI dependencies aligned. @hoogvliets is explicit that this PR is structured for auditability, not independent execution - it depends on the backend (#131) and database (#130) sibling PRs for full runtime behavior.