added custom problem id and endless mode

Author: Edwin Chan

docs/admin-guide.md

@@ -11,6 +11,8 @@
 2. Fill out the set title, slug, topic tags, and settings.
    - **Order ID:** If you leave the `order` as `0` or blank, the system automatically assigns the next available number.
 3. Add problems one-by-one, including statement, answer type, and answer key.
+   - Each problem has an **ID** field (the short label shown to students, e.g. `1`, `2a`, `A1`). It accepts any text — letters, digits, or a mix. Problems are sorted by ASCII order of their ID.
+   - If you leave it as-is, new problems default to sequential numbers (`1`, `2`, `3`, …).
 4. For each problem, use the **LaTeX / HTML** toggle next to the statement field.
    - Use **LaTeX** for `$...$`/`$$...$$` style input.
    - Use **HTML** for content that includes tags like `<math>...</math>`.

docs/import-format.md

@@ -24,23 +24,23 @@ Practice mode only shows tags that belong to more than 10 published questions.
   "videoUrl": "https://example.com/video",
   "problems": [
     {
-      "number": 1,
+      "number": "1",
       "statement": "Solve for x: 2x = 4",
       "answerType": "integer",
       "answerKey": "2",
       "points": 1,
       "topicTags": ["algebra", "linear-equations"]
     },
     {
-      "number": 2,
+      "number": "2",
       "statement": "Give the exact value of sqrt(2).",
       "answerType": "expression",
       "answerKey": "sqrt(2)",
       "acceptedAnswers": ["2^0.5"],
       "topicTags": ["surds"]
     },
     {
-      "number": 3,
+      "number": "3",
       "statement": "State the parity of 17.",
       "answerType": "exact",
       "answerKey": "odd"
@@ -79,7 +79,7 @@ Each entry in `problems` defines one question.
 
 | Field | Type | Required | Default | Notes |
 | :--- | :--- | :--- | :--- | :--- |
-| `number` | integer | No | array index + 1 | Question number. |
+| `number` | string | No | array index + 1 | Question identifier. Supports alphanumeric values like `"1"`, `"1a"`, `"A1"`, `"Q3b"`. Integer values are also accepted and coerced to strings. Ordered by ASCII value. |
 | `statement` | string | No | `""` | Problem statement. LaTeX is allowed. |
 | `statementFormat` | string | No | inherits top-level `statementFormat` | One of `"LATEX"` or `"HTML"`. Use `"HTML"` for content containing tags like `<math>...</math>`. |
 | `answerKey` | string | Yes* | - | Primary correct answer. |
@@ -129,6 +129,9 @@ Per-problem `topicTags` drive Practice mode.
 
 - File size limit: 5 MB
 - The uploaded file must be valid JSON.
-- Duplicate problem numbers are rejected.
+- Duplicate problem numbers (IDs) are rejected.
+- Problem IDs can be any non-empty string — letters, digits, or a mix (e.g. `"1a"`, `"A1"`, `"Q3b"`).
+- When problem IDs are omitted, they default to `"1"`, `"2"`, `"3"`, etc.
+- Problems are sorted by ASCII value of their ID string.
 - Each problem must include an answer.
 - Invalid answer types are rejected.