Merge the deduplication and dead-code removal pass

Consolidates duplicated logic behind shared implementations — paginated
grid tabs, focus chrome, cached remote stores, sheet selection columns,
the server artifact store and a test fixture layer — and removes code
that had become unreachable. Net reduction of about 5,500 lines with no
behaviour change.

Where a fix had landed separately in code that moved into a shared
helper, the fix was re-applied inside the helper rather than left behind
in the copy that went away.

.github/actions/setup-flutter-git/action.yml

@@ -0,0 +1,46 @@
+name: Set up Flutter from git
+description: >-
+  Clone the pinned Flutter SDK from its release tag and put it on PATH, for
+  runners without a published archive. Flutter ships no windows-arm64 SDK, so
+  subosito/flutter-action cannot resolve the release for arm64 (no arm64 entry
+  in the stable manifest) and `channel: master` would clone master HEAD, whose
+  engine is not the patched revision install-patched-engine.ps1 asserts
+  (4c525dac). This file is the only place that pin lives: the tag is fetched so
+  the SDK reports its own version, then verified against the immutable commit
+  and against the version Flutter itself reports, so a moved tag fails the job
+  instead of quietly changing SDKs.
+
+runs:
+  using: composite
+  steps:
+    - name: Clone Flutter from its immutable commit
+      shell: pwsh
+      run: |
+        $version = "3.44.0"
+        $expectedCommit = "559ffa3f75e7402d65a8def9c28389a9b2e6fe42"
+        $root = "$env:RUNNER_TEMP\flutter"
+        git init $root
+        git -C $root remote add origin https://github.com/flutter/flutter.git
+        git -C $root fetch --depth 1 origin "refs/tags/${version}:refs/tags/${version}"
+        git -C $root checkout --detach "refs/tags/$version"
+        $actualCommit = git -C $root rev-parse HEAD
+        if ($LASTEXITCODE -ne 0 -or $actualCommit -ne $expectedCommit) {
+          throw "Flutter $version resolved to $actualCommit, expected $expectedCommit"
+        }
+        "$root\bin" | Out-File -FilePath $env:GITHUB_PATH -Append -Encoding utf8
+        & "$root\bin\flutter.bat" --version
+        if ($LASTEXITCODE -ne 0) {
+          throw "Unable to bootstrap the Flutter SDK"
+        }
+        $versionOutput = & "$root\bin\flutter.bat" --version --machine
+        if ($LASTEXITCODE -ne 0) {
+          throw "Unable to resolve the Flutter SDK version"
+        }
+        $versionJson = $versionOutput -join "`n"
+        if ([string]::IsNullOrWhiteSpace($versionJson)) {
+          throw "Flutter did not report machine-readable version JSON"
+        }
+        $reportedVersion = ($versionJson | ConvertFrom-Json).frameworkVersion
+        if ($reportedVersion -ne $version) {
+          throw "Flutter reported version $reportedVersion, expected $version"
+        }