[feature] File-based group creation / Spliit-JSON-Import (core, formats, server, UI)

[Uli-Z]

This PR introduces a lean, extensible import architecture and wires the Spliit‑JSON format as the first adapter. Goal: reliable import/export as a core capability to ensure data sovereignty and smooth onboarding.

Motivation

• Import/Export is central: users should control their data and move it between instances.
• Onboarding from Splitwise should be simple and lossless (no balance drift) and feel reliable.
• A long-requested Splitwise import (refs #22) needs a solid, generic import foundation, not a one-off integration.
• To accelerate the discussion, a working prototype was built. With a lot of help of AI, it works. But I’m not a web developer — a thorough review is welcome.

Scope in this PR

• Core layer: import models, registry, and file pipeline under src/lib/imports/**.
• Formats: Spliit‑JSON v1 and a Debug format; automatic detection via looksLike + confidence (highest wins; deterministic tie-breaks).
• Server: TRPC procedures for preview/start/chunk/cancel/finalize under groups/import.
• UI: import dialog (analyze → preview → import) with progress and cancel; import is enabled only after a successful analysis.
• Integration: “Import from file” as a submenu next to Create; “Add by URL” is unchanged vs trunk.
• Large flows: progress UI and user-initiated cancel are included.

Decisions

• Automatic format detection keeps the UI simple and resilient; explicit user selection would add friction, and future versions of the same tools will vary in structure.
• Focus on unobtrusive integration: the new import flow sits next to existing create actions without altering the established UI patterns.

Future work

• Splitwise adapter (multi-language), more formats.

Demo