fix: lines-tab theme hint, folder reorder arrows, xTMode import

[kirillDevPro]

What & why

Three small user-reported gaps, one commit each:

• Settings → Lines says nothing about editing only the active theme's line set, so a theme switch reads as "my settings are gone". A second hint now names the two sets (Light / Dark+Graphite), points at the General tab for the theme, and says Save writes cfg/orders.toml.
• Strategies tree: the up/down arrows went dark for a selected folder. The core stores strategy order only, so a non-empty folder now moves as one contiguous block of its strategies (recursively), sent with the same full-order command; an empty folder has no position on the wire and stays disabled with a tooltip saying so.
• Moonbot import: the xTMode (×10 scale) flag of the Trading block was never read, so imported S1–S6 came out ten times smaller than what Moonbot shows. The importer now reads the flag through the pinned moonproto::shared_config decoder, switches the group to the extended TP mode with the main TP canonicalized the way the toolbar's ×10 checkbox does (0 → 100, so the exit generation survives canonicalize()), imports S1–S6 as buffer value ×10, and shows the resulting mode/TP as its own preview row. Without the flag the import is unchanged.

Plus a comment-only pass over the touched code.

How to verify

• cargo fmt --all -- --check clean; cargo test --workspace green (new unit tests: folder block move / nested / loose sibling / empty folder / boundary; xTMode on with default group → Extended + TP 100 + S×10, on with TP 250 → stays 250, off → plan identical to before).
• Debug exe: Settings → Lines shows the new hint under the existing one; Strategies → select a non-empty folder → both arrows enable, core root keeps them disabled.
• Import path checked by tests only — no Moonbot buffer with xTMode on was available on this machine.

Fixes #538
Fixes #524
Fixes #527