feat(studyset): the screens behind a studyset are its path

Deeper studysets, on iOS and Android alike (decided 2026-10-11, after
Material 3's and Apple's guidelines, Android's principles of navigation and
Nielsen Norman Group's on breadcrumbs):

- The screens behind a studyset are its path: below "Japanese › Grammar ›
  Tae Kim's Grammar Guide" lie Grammar's screen, then Japanese's, then the
  library. A topic at any depth opens with the levels between pushed
  beneath it; the path's steps go up by popping to that level's page (they
  replaced the location before, so back could skip levels); a studyset
  opened from elsewhere gets its levels beneath it. Back, Android's
  predictive back, iOS's swipe from the edge and the path all lead one
  level up.
- The path's steps are targets 48 dp high; a long press on back lists the
  levels above, the nearest first and the library last, as iOS's back
  buttons do.
- Below expanded windows the topics open in a sheet from "3 topics ▾" at
  the end of the path's line, where Vaia puts "All topics"; no drawer,
  whose swipe from the edge is iOS's way back and which Material 3
  Expressive deprecates. Topics with topics of their own unfold them, at
  any depth, in the sheet and the side column alike.
- The navigation bar steps aside inside a studyset (its screen, notes,
  paths and dictionary): Material 3 puts it on the top-level screens and
  toolbars below, never both. Apple's guidelines keep the tab bar, lest
  people forget where they are; the path, starting at Library, says where.
  A strip as high as the system's bar keeps those screens clear of it.
- The bar at the bottom keeps Add material and Study.

A trial for review, on its own branch, on top of the floating navigation
bar.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -317,6 +317,10 @@
   "studysetReorderFiles": "Reorder files",
   "studysetReorderDone": "Done",
   "studysetOpenTopics": "Show topics",
+  "studysetTopicCount": "{count, plural, =1{1 topic} other{{count} topics}}",
+  "@studysetTopicCount": {"placeholders": {"count": {"type": "int"}}},
+  "studysetShowInnerTopics": "Show the topics inside",
+  "studysetHideInnerTopics": "Hide the topics inside",
   "studysetTopics": "Topics",
   "studysetAllTopics": "All topics",
   "studysetNewSubStudyset": "New sub-studyset",