diff --git a/changes/2026-03-22-subtitle-sidebar-config.md b/changes/2026-03-22-subtitle-sidebar-config.md
deleted file mode 100644
index 89f18d9..0000000
--- a/changes/2026-03-22-subtitle-sidebar-config.md
+++ /dev/null
@@ -1,5 +0,0 @@
-type: changed
-area: subtitle sidebar
-
-- Added subtitle sidebar state and behavior updates, including startup-auto-open controls and resume positioning improvements.
-- Fixed subtitle prefetch and embedded overlay passthrough sync between sidebar and overlay subtitle rendering.
diff --git a/src/shared/log-files.test.ts b/src/shared/log-files.test.ts
new file mode 100644
index 0000000..797c482
--- /dev/null
+++ b/src/shared/log-files.test.ts
@@ -0,0 +1,65 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import {
+  appendLogLine,
+  pruneLogFiles,
+  resolveDefaultLogFilePath,
+} from './log-files';
+
+test('resolveDefaultLogFilePath uses app prefix by default', () => {
+  const resolved = resolveDefaultLogFilePath('app', {
+    platform: 'linux',
+    homeDir: '/home/tester',
+  });
+
+  assert.equal(
+    resolved,
+    path.join(
+      '/home/tester',
+      '.config',
+      'SubMiner',
+      'logs',
+      `app-${new Date().toISOString().slice(0, 10)}.log`,
+    ),
+  );
+});
+
+test('pruneLogFiles removes logs older than retention window', () => {
+  const logsDir = fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-log-prune-'));
+  const stalePath = path.join(logsDir, 'app-old.log');
+  const freshPath = path.join(logsDir, 'app-fresh.log');
+  fs.writeFileSync(stalePath, 'stale\n', 'utf8');
+  fs.writeFileSync(freshPath, 'fresh\n', 'utf8');
+  const now = new Date('2026-03-22T12:00:00.000Z');
+  fs.utimesSync(stalePath, new Date('2026-03-01T12:00:00.000Z'), new Date('2026-03-01T12:00:00.000Z'));
+  fs.utimesSync(freshPath, new Date('2026-03-21T12:00:00.000Z'), new Date('2026-03-21T12:00:00.000Z'));
+
+  try {
+    pruneLogFiles(logsDir, { retentionDays: 7, now });
+
+    assert.equal(fs.existsSync(stalePath), false);
+    assert.equal(fs.existsSync(freshPath), true);
+  } finally {
+    fs.rmSync(logsDir, { recursive: true, force: true });
+  }
+});
+
+test('appendLogLine trims oversized logs to newest bytes', () => {
+  const logsDir = fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-log-trim-'));
+  const logPath = path.join(logsDir, 'app.log');
+
+  try {
+    appendLogLine(logPath, '012345678901234567890123456789', { maxBytes: 48, retentionDays: 30 });
+    appendLogLine(logPath, 'abcdefghijabcdefghijabcdefghij', { maxBytes: 48, retentionDays: 30 });
+
+    const content = fs.readFileSync(logPath, 'utf8');
+    assert.match(content, /\[truncated older log content\]/);
+    assert.match(content, /abcdefghij/);
+    assert.ok(Buffer.byteLength(content) <= 48);
+  } finally {
+    fs.rmSync(logsDir, { recursive: true, force: true });
+  }
+});
diff --git a/src/shared/log-files.ts b/src/shared/log-files.ts
new file mode 100644
index 0000000..318ed40
--- /dev/null
+++ b/src/shared/log-files.ts
@@ -0,0 +1,123 @@
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+
+export type LogKind = 'app' | 'launcher' | 'mpv';
+
+export const DEFAULT_LOG_RETENTION_DAYS = 7;
+export const DEFAULT_LOG_MAX_BYTES = 10 * 1024 * 1024;
+
+const TRUNCATED_MARKER = '[truncated older log content]\n';
+const prunedDirectories = new Set<string>();
+
+export function resolveLogBaseDir(options?: {
+  platform?: NodeJS.Platform;
+  homeDir?: string;
+  appDataDir?: string;
+}): string {
+  const platform = options?.platform ?? process.platform;
+  const homeDir = options?.homeDir ?? os.homedir();
+  return platform === 'win32'
+    ? path.join(options?.appDataDir?.trim() || path.join(homeDir, 'AppData', 'Roaming'), 'SubMiner')
+    : path.join(homeDir, '.config', 'SubMiner');
+}
+
+export function resolveDefaultLogFilePath(
+  kind: LogKind = 'app',
+  options?: {
+    platform?: NodeJS.Platform;
+    homeDir?: string;
+    appDataDir?: string;
+    now?: Date;
+  },
+): string {
+  const date = (options?.now ?? new Date()).toISOString().slice(0, 10);
+  return path.join(resolveLogBaseDir(options), 'logs', `${kind}-${date}.log`);
+}
+
+export function pruneLogFiles(
+  logsDir: string,
+  options?: {
+    retentionDays?: number;
+    now?: Date;
+  },
+): void {
+  const retentionDays = options?.retentionDays ?? DEFAULT_LOG_RETENTION_DAYS;
+  if (!Number.isFinite(retentionDays) || retentionDays <= 0) return;
+
+  let entries: string[];
+  try {
+    entries = fs.readdirSync(logsDir);
+  } catch {
+    return;
+  }
+
+  const cutoffMs = (options?.now ?? new Date()).getTime() - retentionDays * 24 * 60 * 60 * 1000;
+  for (const entry of entries) {
+    const candidate = path.join(logsDir, entry);
+    let stats: fs.Stats;
+    try {
+      stats = fs.statSync(candidate);
+    } catch {
+      continue;
+    }
+    if (!stats.isFile() || !entry.endsWith('.log') || stats.mtimeMs >= cutoffMs) continue;
+    try {
+      fs.rmSync(candidate, { force: true });
+    } catch {
+      // ignore cleanup failures
+    }
+  }
+}
+
+function maybePruneLogDirectory(logPath: string, retentionDays: number): void {
+  const logsDir = path.dirname(logPath);
+  const key = `${logsDir}:${new Date().toISOString().slice(0, 10)}:${retentionDays}`;
+  if (prunedDirectories.has(key)) return;
+  pruneLogFiles(logsDir, { retentionDays });
+  prunedDirectories.add(key);
+}
+
+function trimLogFileToMaxBytes(logPath: string, maxBytes: number): void {
+  if (!Number.isFinite(maxBytes) || maxBytes <= 0) return;
+
+  let stats: fs.Stats;
+  try {
+    stats = fs.statSync(logPath);
+  } catch {
+    return;
+  }
+  if (stats.size <= maxBytes) return;
+
+  try {
+    const buffer = fs.readFileSync(logPath);
+    const marker = Buffer.from(TRUNCATED_MARKER, 'utf8');
+    const tailBudget = Math.max(0, maxBytes - marker.length);
+    const tail =
+      tailBudget > 0 ? buffer.subarray(Math.max(0, buffer.length - tailBudget)) : Buffer.alloc(0);
+    fs.writeFileSync(logPath, Buffer.concat([marker, tail]));
+  } catch {
+    // ignore trim failures
+  }
+}
+
+export function appendLogLine(
+  logPath: string,
+  line: string,
+  options?: {
+    retentionDays?: number;
+    maxBytes?: number;
+  },
+): void {
+  const retentionDays = options?.retentionDays ?? DEFAULT_LOG_RETENTION_DAYS;
+  const maxBytes = options?.maxBytes ?? DEFAULT_LOG_MAX_BYTES;
+
+  try {
+    fs.mkdirSync(path.dirname(logPath), { recursive: true });
+    maybePruneLogDirectory(logPath, retentionDays);
+    fs.appendFileSync(logPath, `${line}\n`, { encoding: 'utf8' });
+    trimLogFileToMaxBytes(logPath, maxBytes);
+  } catch {
+    // never break runtime due to logging sink failures
+  }
+}