Area/device-scoped activity log on the device tree

[iTerminate]

Motivation

Yesterday's v-scope-cycle work (#21, #27) briefly grew a widened log scope that expanded a whole
list's entities out to every sibling entity on every device backing them — effectively an
area-wide log. That's a legitimate view, but it doesn't belong hanging off an entity list or a
plotted graph: it belongs where an area is an actual node, i.e. the device tree
(src/hatty/ui/device_tree_screen.py), whose area grouping mode already walks
area → device → entity. The over-widened scope has been removed from both the main screen's and
the fullscreen graph's v cycles (see the branch fixing #27's fallout) — this issue is the
follow-up to build it properly, scoped to the tree.

Scope

• Open the existing ActivityLogPanel from a device node: that device's entities +
  its device_id.
• Open it from an area node: every entity of every device in the area + those device_ids.

Reusable pieces

• HACLI._device_ids_for_entities, _cap_log_scope, _open_log_panel,
  hatty.logbook.normalize_log_entries — all already used by the main screen's and fullscreen
  graph's activity logs.
• The main-screen _expand_to_device_entity_ids helper (entity-set → device siblings) was removed
  as part of the v-cycle fix but is recoverable from that commit's history — it's exactly the
  area-expansion helper this needs, generalized from "a list of entities" to "a tree node".

Constraint

a, i, and v are all already bound on the device tree screen
(src/hatty/ui/device_tree_screen.py:411-431, none of them log-related today), so this needs a
free key rather than reusing those.