Use Pierre Diffs edit mode for in-app file editing and persist unsaved drafts

[danielss-dev]

Motivation

• Integrate the new Pierre / diffs.com edit mode so the File view uses the library's built-in editor surface instead of the legacy textarea-based HighlightedEditor for working-tree edits.
• Preserve users' unsaved changes while navigating the app (tabs, panes, repos) but keep writes explicit (save button / Mod+S) to avoid implicit disk writes.
• Align package metadata and docs with the new edit-mode integration.

Description

• Reworked ui/src/views/FileView.tsx to mount Pierre's edit surface: wrapped the file surface in an EditProvider and render an editable Pierre File when canEditFileContent is true, removing the in-repo HighlightedEditor implementation.
• Added in-memory draft persistence keyed per working-tree file via ui/src/lib/fileEditing.ts::fileDraftKey and a sessionFileDrafts map so unsaved drafts survive navigation but are not auto-saved.
• Wire the editor's onChange and save flow to maintain/delete the session draft map on edits and successful saves (keeps explicit save semantics).
• Added a small ambient declaration ui/src/vite-pierre-edit.d.ts to type the @pierre/diffs/edit entry used at build time.
• Bumped UI dependency on @pierre/diffs to ^1.3.2 and updated pnpm-lock.yaml.
• Updated tests and docs: ui/src/lib/fileEditing.test.ts extended to cover fileDraftKey; updated README.md, TASKS.md, ROADMAP.md, and website/docs/work.md to describe edit-mode and unsaved-draft behavior.
• Kept the previous save API (tauri.repoFileWrite) and stale-write rejection behavior unchanged.

Testing

• Ran TypeScript checks with pnpm --filter ./ui exec tsc --noEmit and the UI typecheck passed.
• Ran unit tests with pnpm --filter ./ui exec vitest run src/lib/fileEditing.test.ts and the added test passed (1 file, 3 tests).
• Ran git diff --check (no whitespace/errors).
• Attempted cargo check -p strand-core -p strand-tauri; the Rust build failed in this environment due to missing system glib-2.0 pkg-config dependency (environment issue), not code regressions.

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Codex Task