Fix kin-distance metric: shortest descent from direct lineage

Replaces the previous "depth in generations along the strict
lineal chain" definition (which excluded siblings, aunts, cousins
entirely) with the metric the user actually wants: the number of
descent-steps separating the target from the recipient's closest
direct ancestor or descendant.

Examples relative to the recipient:
- sibling:        1  (parent → sibling)
- great-aunt:     1  (great-grandparent → great-aunt)
- nephew:         2  (parent → sibling → nephew)
- first cousin:   2  (grandparent → aunt → cousin)
- second cousin:  3
- ego, parents, grandparents, ..., children, ..., great-greats: 0
- own spouse, step-parents, brothers-in-law: inherit partner's
  distance (so spouse-of-distance-1 is also distance 1)

Implementation:
- Anchor set seeded with R's direct ancestors + R + direct
  descendants (capped at 25 generations to bound runaway data).
- Multi-source BFS expanding by descent only.
- Spouse propagation at every level so a person and their
  spouse always share the same distance.
- Memoised per (recipient xref, max distance).

Tree preference key and range kept (NEWSLETTER_LINEAL_DEPTH,
0–10, default 3); only the semantics and the user-facing label
+ help text change, with concrete examples in both English and
German.

resources/views/admin.phtml

@@ -132,7 +132,7 @@ use Illuminate\Support\Collection;
 
                 <div class="row mb-3">
                     <label class="col-sm-3 col-form-label" for="lineal-<?= $id ?>">
-                        <?= I18N::translate('Detailed view depth (generations)') ?>
+                        <?= I18N::translate('Detailed view distance') ?>
                     </label>
                     <div class="col-sm-9">
                         <input class="form-control" type="number" style="max-width: 18rem;"
@@ -141,7 +141,7 @@ use Illuminate\Support\Collection;
                                min="<?= Configuration::MIN_LINEAL_DEPTH ?>"
                                max="<?= Configuration::MAX_LINEAL_DEPTH ?>" required>
                         <small class="form-text text-muted">
-                            <?= I18N::translate('Recipients see profile pictures, icons and the timeline only for their own direct ancestors and descendants within this many generations. Everyone else appears as a compact text list at the bottom of each section. Set to 0 to render the whole newsletter as text. Recipients with no linked tree record always see the full detailed view.') ?>
+                            <?= I18N::translate('A person is shown in detail (avatar, icon, timeline) when they sit within this many descent-steps of the recipient\'s direct lineage. Examples relative to the recipient: a sibling is distance 1 (one step down from the recipient\'s parent), a great-aunt is distance 1 (one step down from a great-grandparent), a nephew is distance 2, a first cousin is distance 2. Spouses share their partner\'s distance. Everyone outside this radius appears as a compact text bullet at the bottom of each section. Set to 0 to render the whole newsletter as text; recipients with no linked tree record always see the full detailed view.') ?>
                         </small>
                     </div>
                 </div>