optimize: WeChat QR code auth flow and channel reliability

- Generate QR code server-side when iLink API returns SPA page URL
  (added qrcode + image crates for PNG generation)
- Strip bot_token from frontend response (new WeixinQrcodeStatusPublic type)
- Add request timeouts and shared HTTP client for QR code endpoints
- Fix TOCTOU race on reply_context double-lock (single lock scope)
- Extract do_send() helper to deduplicate sendmessage logic; resend now
  checks ret field for context expiry instead of HTTP status only
- Cap pending_messages buffer at 50 to prevent unbounded memory growth
- Generate stable X-WECHAT-UIN per backend instance instead of per request
- Extract ILINK_CHANNEL_VERSION constant (was hardcoded in 4 places)
- Add 5-minute client-side QR expiry timeout in frontend dialog
- Track consecutive polling errors and show warning after 3 failures
- Stabilise onAuthSuccess/onClose callback refs to prevent polling restarts
- Replace dead i18n key weixinOpenQrcode with weixinPollError

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/Cargo.lock

@@ -615,6 +615,12 @@ version = "1.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1fd0f2584146f6f2ef48085050886acf353beff7305ebd1ae69500e27c67f64b"
 
+[[package]]
+name = "byteorder-lite"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8f1fe948ff07f4bd06c30984e69f5b4899c516a3ef74f34df92a2df2ab535495"
+
 [[package]]
 name = "bytes"
 version = "1.11.1"
@@ -860,12 +866,14 @@ dependencies = [
  "flate2",
  "futures",
  "futures-util",
+ "image",
  "keyring",
  "kill_tree",
  "mac-notification-sys",
  "notify",
  "portable-pty",
  "prost",
+ "qrcode",
  "rand 0.8.5",
  "regex",
  "reqwest 0.12.28",
@@ -2465,7 +2473,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3e795dff5605e0f04bff85ca41b51a96b83e80b281e96231bcaaf1ac35103371"
 dependencies = [
  "byteorder",
- "png",
+ "png 0.17.16",
 ]
 
 [[package]]
@@ -2576,6 +2584,19 @@ dependencies = [
  "icu_properties",
 ]
 
+[[package]]
+name = "image"
+version = "0.25.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "85ab80394333c02fe689eaf900ab500fbd0c2213da414687ebf995a65d5a6104"
+dependencies = [
+ "bytemuck",
+ "byteorder-lite",
+ "moxcms",
+ "num-traits",
+ "png 0.18.1",
+]
+
 [[package]]
 name = "indexmap"
 version = "1.9.3"
@@ -3196,6 +3217,16 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "moxcms"
+version = "0.8.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bb85c154ba489f01b25c0d36ae69a87e4a1c73a72631fc6c0eb6dde34a73e44b"
+dependencies = [
+ "num-traits",
+ "pxfm",
+]
+
 [[package]]
 name = "muda"
 version = "0.17.1"
@@ -3211,7 +3242,7 @@ dependencies = [
  "objc2-core-foundation",
  "objc2-foundation",
  "once_cell",
- "png",
+ "png 0.17.16",
  "serde",
  "thiserror 2.0.18",
  "windows-sys 0.60.2",
@@ -4119,6 +4150,19 @@ dependencies = [
  "miniz_oxide",
 ]
 
+[[package]]
+name = "png"
+version = "0.18.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "60769b8b31b2a9f263dae2776c37b1b28ae246943cf719eb6946a1db05128a61"
+dependencies = [
+ "bitflags 2.10.0",
+ "crc32fast",
+ "fdeflate",
+ "flate2",
+ "miniz_oxide",
+]
+
 [[package]]
 name = "polling"
 version = "3.11.0"
@@ -4340,6 +4384,21 @@ dependencies = [
  "syn 1.0.109",
 ]
 
+[[package]]
+name = "pxfm"
+version = "0.1.28"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b5a041e753da8b807c9255f28de81879c78c876392ff2469cde94799b2896b9d"
+
+[[package]]
+name = "qrcode"
+version = "0.14.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d68782463e408eb1e668cf6152704bd856c78c5b6417adaee3203d8f4c1fc9ec"
+dependencies = [
+ "image",
+]
+
 [[package]]
 name = "quick-xml"
 version = "0.37.5"
@@ -6263,7 +6322,7 @@ dependencies = [
  "ico",
  "json-patch",
  "plist",
- "png",
+ "png 0.17.16",
  "proc-macro2",
  "quote",
  "semver",
@@ -7014,7 +7073,7 @@ dependencies = [
  "objc2-core-graphics",
  "objc2-foundation",
  "once_cell",
- "png",
+ "png 0.17.16",
  "serde",
  "thiserror 2.0.18",
  "windows-sys 0.60.2",