feat(studyset): learning paths are material, the study plan refers to them

A learning path is material too, as its flashcards, notes and files are,
and the study plan only refers to it (decided 2026-10-13), as it will to
an exam's date. Material's tabs are now Paths, Flashcards, Notes and
Files, the paths first as the way through the rest, with Reorder beside
them; opened here for the first time, Material shows the first tab with
something in it. Study plan keeps how many paths are finished and Next
up, which names the next step of each path under way ("Step 3 of 6: が"),
or offers to start the first path not finished when none is, and lists
how far every path has come, its steps done in a bar, each opening the
path. The path list's state now knows each path's steps done and next
step.

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

halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -2049,6 +2049,14 @@
   "@planReview": {"placeholders": {"count": {"type": "int"}}},
   "planContinue": "Continue “{title}”",
   "@planContinue": {"placeholders": {"title": {"type": "String"}}},
+  "planStart": "Start “{title}”",
+  "@planStart": {"placeholders": {"title": {"type": "String"}}},
+  "planStepOf": "Step {number} of {total}",
+  "@planStepOf": {"placeholders": {"number": {"type": "int"}, "total": {"type": "int"}}},
+  "planStepOfTitled": "Step {number} of {total}: {title}",
+  "@planStepOfTitled": {"placeholders": {"number": {"type": "int"}, "total": {"type": "int"}, "title": {"type": "String"}}},
+  "planStepsDone": "{total, plural, =1{{done} of 1 step} other{{done} of {total} steps}}",
+  "@planStepsDone": {"placeholders": {"done": {"type": "int"}, "total": {"type": "int"}}},
   "planNothing": "Nothing due right now.",
   "planLearningPaths": "Learning paths",
   "jellyfinTitle": "Jellyfin",