feat(paths): confetti for a finished path, and an import's progress

Finishing a path bursts confetti over the view saying so: pieces in the
palette's brightest colours fall once from the top, turning, and fade
out in under two seconds, so the screen settles. Drawn by hand rather
than with a package; nothing falls with animations switched off on the
device.

Importing paths into a studyset shows its Paths tab at once, and above
the paths, as they come in, how far the import has come: which path of
how many, by its title, and a bar filling with every path and step
stored. Before, the paths appeared one by one in whatever tab showed,
with nothing saying how many were still to come.

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

@@ -112,6 +112,10 @@
   "pathImportFailed": "Couldn't import this file. {reason}",
   "@pathImportFailed": {"placeholders": {"reason": {"type": "String"}}},
   "pathImportFileFailed": "Couldn't import {file}. {reason}",
+  "pathImportReading": "{count, plural, =1{Reading the file…} other{Reading {count} files…}}",
+  "@pathImportReading": {"placeholders": {"count": {"type": "int"}}},
+  "pathImportStoring": "{count, plural, =1{Importing “{title}”…} other{Importing path {index} of {count}: “{title}”…}}",
+  "@pathImportStoring": {"placeholders": {"index": {"type": "int"}, "count": {"type": "int"}, "title": {"type": "String"}}},
   "@pathImportFileFailed": {"placeholders": {"file": {"type": "String"}, "reason": {"type": "String"}}},
   "pathsImported": "{steps, plural, =0{Imported {count} paths without steps.} =1{Imported {count} paths with 1 step.} other{Imported {count} paths with {steps} steps.}}",
   "@pathsImported": {"placeholders": {"count": {"type": "int"}, "steps": {"type": "int"}}},