nix/hive-matrix: run dart compile js from build CWD so package_config resolves (#685 fixup)

mara's first deploy hit:

    Error: Couldn't resolve the package 'matrix' in 'package:matrix/matrix.dart'.
    /nix/store/k9j8ns45fz7rpjp6rzk33ydjng67pgm0-source/web/native_executor.dart:1:8:
    Error: Not found: 'package:matrix/matrix.dart'

Root cause: `dart compile js` walks up from the source file's dir to
find `.dart_tool/package_config.json`. My previous postInstall passed
`$src/web/native_executor.dart` — pointing dart at the unpacked nix
source, which has no `.dart_tool/` (pub-get wrote it to the build CWD,
not the read-only store path).

Fix: use a relative path `web/native_executor.dart`. nixpkgs's
buildFlutterApplication leaves CWD at the source root in postInstall
(its installPhase is just `cp -r build/web "$out"` with no `cd`
first — see `pkgs/development/compilers/flutter/build-support/
build-flutter-application.nix`), so the relative path walks up from
`web/` to the build CWD where pub-get's package_config lives.

Verified by `nix eval`; full closure build pending operator deploy.
Followup to #697 (the original fix; merged but mara's deploy then
surfaced this regression).

nix/modules/hive-matrix.nix

@@ -126,9 +126,26 @@ let
         # The flutter web bootstrap loads this from /matrix/native_executor.js
         # at startup; without it, main.dart.js logs a network-error and
         # the SPA renders blank (see #643 for the symptom).
+        #
+        # `dart compile js` needs `.dart_tool/package_config.json` to
+        # resolve `package:matrix/...` and the rest of fluffychat's
+        # `pubspec.lock` deps. buildFlutterApplication's pub-get step
+        # writes that file to the build CWD (the unpacked source dir),
+        # not to `$src` (the read-only nix store path). So we must use
+        # a relative path that walks up from `web/` to the build CWD
+        # where pub-get's package_config lives — pointing at
+        # `$src/web/native_executor.dart` walks up to `$src/`, finds
+        # no `.dart_tool/`, and fails with `Couldn't resolve the
+        # package 'matrix'` (mara's first build attempt on #685).
+        #
+        # nixpkgs's buildFlutterApplication leaves CWD at the source
+        # root for postInstall (see `pkgs/development/compilers/flutter/
+        # build-support/build-flutter-application.nix` — installPhase
+        # is `cp -r build/web "$out"` with no `cd` first). So `web/...`
+        # resolves correctly here.
         ${pkgs.flutter341.dart}/bin/dart compile js \
           -o $out/native_executor.js \
-          $src/web/native_executor.dart
+          web/native_executor.dart
 
         # #685: install Imaging.{js,wasm} built from the native_imaging
         # dart package's C source via emscripten (see