feat: files in an order of one's own, with Reorder beside the tabs

A PDF now has a position among its studyset's files, as notes and paths
have: a fractional index in a column of its own (so moving it on one
device and renaming it on another don't overwrite each other), last
writer wins. The migration adds it to pdf_source on the server and on
each device with 'a0', so PDFs from before stand in the order they were
added (ties sort by id), and new ones go at the end.

The Files tab lists them in that order by default (Custom order, first
in the Sort menu; newest first was the default before). With two files
or more, Reorder beside the tabs shows the custom order whatever they
were sorted by, a grip in place of each file's options and the Sort menu
put away, until Done.

The next release changes synced tables anyway (card.buriedAt, since
v0.2.2), so older apps have to update for that already.

Also: card.position's doc no longer speaks of moving cards.

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_client/lib/migrations/20261008165051407-pdf-position/migration.json

@@ -0,0 +1,56 @@
+{
+  "__className__": "serverpod.DatabaseMigration",
+  "actions": [
+    {
+      "__className__": "serverpod.DatabaseMigrationAction",
+      "type": "alterTable",
+      "alterTable": {
+        "__className__": "serverpod.TableMigration",
+        "name": "pdf_source",
+        "schema": "public",
+        "addColumns": [
+          {
+            "__className__": "serverpod.ColumnDefinition",
+            "name": "position",
+            "fieldName": "position",
+            "columnType": 0,
+            "isNullable": false,
+            "columnDefault": "'a0'",
+            "dartType": "String"
+          }
+        ],
+        "deleteColumns": [],
+        "modifyColumns": [],
+        "addIndexes": [
+          {
+            "__className__": "serverpod.IndexDefinition",
+            "indexName": "pdf_source_studyset_idx",
+            "elements": [
+              {
+                "__className__": "serverpod.IndexElementDefinition",
+                "type": 0,
+                "definition": "studysetId"
+              },
+              {
+                "__className__": "serverpod.IndexElementDefinition",
+                "type": 0,
+                "definition": "position"
+              }
+            ],
+            "type": "btree",
+            "isUnique": false,
+            "isPrimary": false
+          }
+        ],
+        "deleteIndexes": [
+          "pdf_source_studyset_idx"
+        ],
+        "addForeignKeys": [],
+        "deleteForeignKeys": [],
+        "warnings": []
+      }
+    }
+  ],
+  "warnings": [],
+  "migrationApiVersion": 1
+}