chore(repo): Add workflow to publish design system gallery

[renefloor]

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CLA

• I have signed the Stream CLA (required).
• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

Description of the pull request

For workflow-dispatch this workflow has to be available on main, when this is merged I'll cherry-pick for the design system branch.

Summary by CodeRabbit

• Chores
  • Adds an automated deployment workflow for the design system gallery: runs on main-design-system updates and via manual trigger, uses the stable Flutter channel, enables concurrent-run cancellation, enforces a 10-minute job timeout, and publishes the gallery to the project's pages site.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a new GitHub Actions workflow to build and deploy the Flutter design system gallery to GitHub Pages on pushes to the main-design-system branch and on manual dispatch.

Changes

Cohort / File(s)	Summary
GitHub Actions Workflow .github/workflows/publish_gallery.yml	New workflow "Publish Design System Gallery" that triggers on pushes to main-design-system and workflow_dispatch; checks out code, configures git, fetches gh-pages, sets Flutter channel, and deploys from apps/design_system_gallery to gh-pages with baseHref /stream-core-flutter/.

Sequence Diagram(s)

sequenceDiagram
  autonumber
  participant GH as GitHub Actions
  participant Runner as CI Runner
  participant Checkout as actions/checkout
  participant FlutterSetup as subosito/flutter-action
  participant Deploy as bluefireteam/flutter-gh-pages
  participant Pages as GitHub Pages

  GH->>Runner: trigger build_and_deploy_gallery (push or manual)
  Runner->>Checkout: checkout repository
  Runner->>Checkout: fetch gh-pages branch
  Runner->>FlutterSetup: set FLUTTER_CHANNEL (stable) & setup Flutter
  Runner->>Deploy: run deploy from apps/design_system_gallery with baseHref /stream-core-flutter/
  Deploy->>Pages: push artifacts to gh-pages branch
  Pages-->>GH: site published

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Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 A little workflow hops with glee,
Building a gallery for all to see,
Flutter set steady, pages take flight,
Designs land softly, morning to night. 🥕✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding a GitHub Actions workflow to publish the design system gallery.
Description check	✅ Passed	The description includes CLA confirmation and testing attestation, but lacks detail on how the workflow functions and how it can be tested.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.39%. Comparing base (81fa8bd) to head (c876e34).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #32   +/-   ##
=======================================
  Coverage   40.39%   40.39%           
=======================================
  Files          53       53           
  Lines        1057     1057           
=======================================
  Hits          427      427           
  Misses        630      630           

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[coderabbitai]

Actionable comments posted: 2

🤖 Fix all issues with AI agents

In @.github/workflows/publish_gallery.yml:
- Around line 25-29: The workflow's "config git" step currently runs git fetch
origin gh-pages:gh-pages which will fail if the gh-pages branch doesn't exist;
update that step to handle the missing-branch case by checking for the remote
branch before fetching and, if absent, create an initial gh-pages branch locally
(or skip the fetch) so the job doesn't fail on first deploy; alternatively
remove the explicit fetch if you confirm the bluefireteam/flutter-gh-pages
action already creates or manages the gh-pages branch.
- Around line 17-23: The workflow job build_and_deploy_gallery is missing a
permissions block needed by the bluefireteam/flutter-gh-pages action to push to
gh-pages; add a permissions: block at the job level (under
build_and_deploy_gallery) granting write access (e.g., contents: write and
pages: write) so the action can push the built site.