Open projects and journals: replace by default, opt-in append with group renumbering (#174)

[jepegit]

Closes #174.

Stacked PR — based on cursor/187-group-names-713a (PR #191 → #190 → #189). Merge in order #189 → #190 → #191 → this one; GitHub retargets the base to main when the previous branch is deleted after its squash merge. The diff shown against the stacked base is this issue only.

What

Every open — saved project, project.json / folder path, batch journal — now has an explicit mode:

• replace (default): clears the loaded cells first, exactly what project open always did silently. The UI now asks for confirmation when cells are loaded (and says when unsaved changes would be lost).
• append: keeps the loaded cells. Incoming group numbers are offset by the highest group already in use (Library.group_offset_for_append()), so a second project's groups 1, 2 land at max+1, max+2 with their internal grouping and their names (name of groups #187) intact — the collision reported in the issue comment. The current project association is kept (Save still writes the project you had open) and the set is marked dirty.

UI

• Project panel: an Append to the loaded cells checkbox (shown once cells exist). The Open / Open path buttons read Append / Append path while it is on; the panel hint explains the default.
• Toast: "Appended “…” — N cells added to the loaded cells."
• The journal tab of Add cells… always appends — "add" is its whole point.

API

• POST /api/projects/open {target, mode?} and POST /api/projects/load-journal {path, mode?} (mode ∈ replace | append, default replace). Open job result carries action: opened | appended.
• The journal job clears the library only after the journal produced cells — a corrupt or empty journal can no longer wipe the loaded set. Replace also drops the active project's cellpy config.
• cellpy project config (cellpy.toml) is switched only when the open actually replaces; an append reads the incoming cells under the current project's settings (documented limitation).

Tests

• tests/test_projects.py: replace default; append keeps cells / renumbers groups / carries names / keeps association; append into empty library; group_offset_for_append; API mode round trip + 422 on unknown mode.
• tests/test_journal.py: journal append renumbers groups; replace is the default; corrupt journal under replace keeps the library.
• tests/test_gui_playwright.py (e2e, local): Append → no confirm, two cells, distinct groups, dirty tag; Open → confirm text, one cell left.

uv run pytest -q -p no:warnings: all green (4 skips, example data). Design notes in .issueflows/04-designs-and-guides/project-refresh-and-import.md; README "Projects on disk" updated.

Walkthrough

Demo cells (groups 1–3) → Open asks "Replace the 3 loaded cells…?" → Cancel keeps them → tick Append → button reads "Append", no dialog, 6 cells, appended groups land at 4 and 6 (named group "Anodes" travels along) → untick → Open confirms and leaves the 3 saved cells.

issue174_open_replace_vs_append_walkthrough.mp4

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