fix(security): anti-replay, 0600 client key, open redirect, supply-chain

Address findings from a repo security review:

- core: add a sliding-window anti-replay filter over the AEAD-authenticated
  sequence in Session (poll_input/poll_frame), closing the input-replay gap the
  data plane previously left to the LAN/VPN trust assumption. 4096-deep window,
  unit-tested; the encrypted loopback suite confirms no false drops.
- clients: write the mTLS client private key 0600 and lock the config dir 0700
  on Unix (it was world-readable at the umask default), re-locking existing
  stores on load. pf-client-core::trust plus the probe's own identity writer.
  Windows keeps the %APPDATA% ACL; Android/Apple already wrap the key.
- web: fix a post-login open redirect — resolve `next` via URL and require it to
  stay same-origin, rejecting `/\evil.com` and tab/encoding variants the old
  `!startsWith("//")` guard missed. Also fixes the dead safeNextPath helper.
- ci: SHA-256-pin the BtbN FFmpeg DLLs bundled into the signed Windows installer
  (were fetched from the rolling `latest` tag unverified); fails closed on a
  re-roll, matching the VB-CABLE gate.
- ci: fail-open fork-guard on the Windows/Apple host-mode PR build jobs that
  share runner labels with the signing jobs. Definitive fix stays server-side
  (Gitea outside-collaborator approval / isolated PR runners) — see the notes.

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

.gitea/workflows/windows-drivers.yml

@@ -30,8 +30,15 @@ on:
 # scripts/ci/ensure-windows-toolchain.ps1, a fast no-op once already present.
 
 jobs:
+  # SECURITY: builds PULL-REQUEST code on the host-mode, persistent `windows-amd64` runner shared with
+  # the release-signing jobs (windows-host.yml / windows-msix.yml). See windows.yml for the full
+  # rationale. Definitive fix is server-side (Gitea outside-collaborator approval + isolated PR
+  # runners); the `if:` is a fail-open backstop that never blocks internal PR CI.
   probe-and-proto:
     runs-on: windows-amd64
+    if: >-
+      github.event_name != 'pull_request' ||
+      github.event.pull_request.head.repo.fork != true
     timeout-minutes: 30
     defaults:
       run:
@@ -108,6 +115,9 @@ jobs:
   # DLL's FORCE_INTEGRITY (/INTEGRITYCHECK) bit — the M0 self-signed-load question.
   driver-build:
     runs-on: windows-amd64
+    if: >-
+      github.event_name != 'pull_request' ||
+      github.event.pull_request.head.repo.fork != true
     timeout-minutes: 45
     defaults:
       run: