fix(mining): harden xmrig updater per adversarial review

Addresses confirmed findings from the multi-lens review of the updater:

- Cancelable + live progress (was: download uncancelable, progress stuck at 0%, closing
  the dialog mid-download blocked the UI thread on the worker join). Wire a libcurl
  CURLOPT_XFERINFOFUNCTION that publishes byte counts and returns abort when cancel() is
  requested; add a Cancel button. The dialog's destructor now aborts the transfer promptly,
  so closing mid-download no longer freezes the UI.
- Graceful "unavailable" instead of a red error on platforms with no published build
  (macOS / ARM): new terminal State::Unavailable rendered neutrally, not as a failure.
- Install-time running guard (TOCTOU): App::isPoolMinerRunning() re-checked in the dialog
  before each install, so a dialog opened before mining started can't replace a live binary.
- Size caps: CURLOPT_MAXFILESIZE on the download and a per-archive-member ceiling before
  decomphressing into memory, to bound an attacker-controlled archive.
- Distinguish a local read failure of the downloaded archive from a checksum mismatch
  (was reported misleadingly as "possible tampering").
- Reword the dialog's verification note to "checked against the release's published SHA-256
  checksum" (integrity, not authenticity — see the signing note below).

Not fixed here (needs your input): WinRing0x64.sys has no per-file hash published, but it is
covered by the verified archive checksum (it is inside the verified zip); and the release is
not cryptographically signed — checksums and binary share one trust root. Adding a pinned-key
ed25519/minisign signature is the real supply-chain hardening and needs an offline signing key
+ a release-process change.

Both variants build; suite passes; live worker re-verified end-to-end on linux-x64.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.h

@@ -197,6 +197,10 @@ public:
     int getXmrigRequestedThreads() const {
         return xmrig_manager_ ? xmrig_manager_->getRequestedThreads() : 0;
     }
+    // True while the pool miner process is live — used to refuse replacing the binary under it.
+    bool isPoolMinerRunning() const {
+        return xmrig_manager_ && xmrig_manager_->isRunning();
+    }
 
     // Mine-when-idle state query
     bool isIdleMiningActive() const { return idle_mining_active_; }