fix: skip cache-busting and wake-up reload for file:// URLs

[jamesvillarrubia]

Problem

When the service worker wakes up on a file:// tab, the console fills with:

Unsafe attempt to load URL file:///.../file.md from frame with URL file:///.../file.md.
'file:' URLs are treated as unique security origins.

and the autoreload poll fails for local files.

Root cause

Two spots assume file:// behaves like http(s)://:

1. background/detect.js — wake-up reload. The onwakeup branch calls chrome.tabs.reload(id) whenever chrome.webRequest is present. But webRequest only fires for http/https navigations — it cannot observe file:// loads (see README, issue Firefox version breaks GitHub #41). So on wake-up, a file:// tab gets a pointless full reload that re-triggers the raw-text load and races the content-script re-inject, surfacing the file: unique-security-origin error.

2. background/xhr.js — autoreload fetch. Every URL — including file:// paths — got ?preventCache= + Date.now() appended. file: URLs aren't cached, and some Chrome versions reject the query string on file: fetches, so the autoreload poll throws and the extension logs res.err.

Fix

• detect.js: guard the onwakeup reload with !win.url.startsWith('file:') so file:// tabs fall through to inject() instead of reloading.
• xhr.js: only append the cache-busting query string for non-file: URLs.

Repro

1. Open a local .md file (file:///...) in Chrome with the extension and Allow access to file URLs enabled.
2. Leave the tab open long enough for the MV3 service worker to suspend (or click "Service Worker → Stop" in chrome://extensions).
3. Reload the tab / trigger the wake-up.
4. Observe the Unsafe attempt to load URL ... 'file:' URLs are treated as unique security origins error in the service-worker console, and (with autoreload on) the xhr fetch error.

After this patch, the wake-up path injects without reloading and the autoreload poll no longer appends a query string to file: URLs.

// background/detect.js
-          if (onwakeup && chrome.webRequest) {
+          if (onwakeup && chrome.webRequest && !win.url.startsWith('file:')) {

// background/xhr.js
-        var res = await fetch(url + '?preventCache=' + Date.now())
+        var bust = url.startsWith('file:') ? '' : '?preventCache=' + Date.now()
+        var res = await fetch(url + bust)

Related: #288, #131.