Detect course roots per branch and add a category layer above them

The scanner no longer assumes courses sit at a fixed depth. It walks
each branch and marks a folder is_course when it holds media directly
or its children read as chapters (01_, 1. , Module 2, Week 3);
everything above that is a category, and every folder still gets a
course_id pointing at its nearest course for cheap rollups. An empty
.course/.category marker file overrides the heuristic per folder.

Also:
- Roll up progress for the whole library in two grouped queries
  instead of one prefix-matched query per course/folder card.
- Store folders/videos/quizzes paths as utf8mb4_bin so folders that
  differ only in case (CCNP Notes vs CCNP NOtes) aren't merged by a
  case-insensitive UNIQUE key.
- migrateLibrarySchema() brings an older install's tables up to date
  at scan time, so there's no separate migration step.
- Library UI gains a categories view alongside shelves/catalog; the
  continue-watching strip's play button now seeks to the saved
  position on open.

Claude-Session: https://claude.ai/code/session_01FfMPJK7TQV18JhFv6yTKev

schema.sql

@@ -31,18 +31,35 @@ CREATE TABLE IF NOT EXISTS app_meta (
 
 -- One row per directory found under VIDEOS_ROOT.
 -- `path` is the relative path from VIDEOS_ROOT, e.g. "Course 1/Intro".
+--
+-- Paths are utf8mb4_bin: they identify a file on a case-sensitive filesystem,
+-- so "CCNP Notes" and "CCNP NOtes" are two directories, not one. Under the
+-- table's default (case-insensitive) collation the UNIQUE key merges them and
+-- the second tree silently overwrites the first.
+--
+-- `is_course` marks the folder the library shows as a course. Course roots sit
+-- at whatever depth the material happens to use — a top-level folder may be a
+-- category (Networking/Cisco/<course>) or a course in its own right — so the
+-- scanner works it out per branch rather than assuming a fixed depth; see
+-- markCourseRoots(). `course_id` is the nearest course at-or-above the folder
+-- (NULL for the categories above every course), which is what makes a course's
+-- rollup one grouped query instead of one query per course.
 CREATE TABLE IF NOT EXISTS folders (
     id         INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
     parent_id  INT UNSIGNED NULL,
     name       VARCHAR(255) NOT NULL,
-    path       VARCHAR(700) NOT NULL UNIQUE,
+    path       VARCHAR(700) COLLATE utf8mb4_bin NOT NULL UNIQUE,
     depth      INT UNSIGNED NOT NULL DEFAULT 0,
     sort_order INT UNSIGNED NOT NULL DEFAULT 0,
+    is_course  TINYINT(1) NOT NULL DEFAULT 0,
+    course_id  INT UNSIGNED NULL,
     is_missing TINYINT(1) NOT NULL DEFAULT 0,
     created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
     CONSTRAINT fk_folders_parent FOREIGN KEY (parent_id)
         REFERENCES folders(id) ON DELETE CASCADE,
-    INDEX idx_folders_parent (parent_id)
+    INDEX idx_folders_parent (parent_id),
+    INDEX idx_folders_course (course_id),
+    INDEX idx_folders_is_course (is_course, is_missing)
 ) ENGINE=InnoDB;
 
 CREATE TABLE IF NOT EXISTS videos (
@@ -50,7 +67,7 @@ CREATE TABLE IF NOT EXISTS videos (
     folder_id        INT UNSIGNED NOT NULL,
     name             VARCHAR(255) NOT NULL,   -- display name, filename without .mp4
     filename         VARCHAR(255) NOT NULL,
-    path             VARCHAR(700) NOT NULL UNIQUE,  -- relative path incl. filename
+    path             VARCHAR(700) COLLATE utf8mb4_bin NOT NULL UNIQUE,  -- relative path incl. filename
     size_bytes       BIGINT UNSIGNED NOT NULL DEFAULT 0,
     duration_seconds FLOAT NULL,              -- filled in by the player on first load
     sort_order       INT UNSIGNED NOT NULL DEFAULT 0,
@@ -69,7 +86,7 @@ CREATE TABLE IF NOT EXISTS quizzes (
     folder_id     INT UNSIGNED NOT NULL,
     name          VARCHAR(255) NOT NULL,   -- filename without the _quiz_solved.txt suffix
     filename      VARCHAR(255) NOT NULL,
-    path          VARCHAR(700) NOT NULL UNIQUE,
+    path          VARCHAR(700) COLLATE utf8mb4_bin NOT NULL UNIQUE,
     num_questions INT UNSIGNED NOT NULL DEFAULT 0,
     sort_order    INT UNSIGNED NOT NULL DEFAULT 0,
     is_missing    TINYINT(1) NOT NULL DEFAULT 0,