docs: tighten public copy, add comparison, templates, and social preview

* docs: tighten README, CONTRIBUTING, and CLA prose

* docs: add platform comparison and README FAQ

* chore: add issue and pull request templates

* chore: add repository social preview image

.github/ISSUE_TEMPLATE/bug_report.yml

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+name: Bug report
+description: Report something that is broken or behaving incorrectly.
+labels: ["bug"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for taking the time to file a bug. Please search existing issues first to avoid duplicates.
+        For security vulnerabilities, do not open a public issue. See SECURITY.md and report privately.
+  - type: textarea
+    id: what-happened
+    attributes:
+      label: What happened
+      description: A clear description of the bug and what you expected instead.
+      placeholder: When I do X, Y happens. I expected Z.
+    validations:
+      required: true
+  - type: textarea
+    id: repro
+    attributes:
+      label: Steps to reproduce
+      description: The exact steps that trigger the problem.
+      placeholder: |
+        1. Go to ...
+        2. Click ...
+        3. See ...
+    validations:
+      required: true
+  - type: dropdown
+    id: deploy-mode
+    attributes:
+      label: Deployment mode
+      description: How is this instance deployed?
+      options:
+        - All-in-One (bundled Caddy)
+        - Behind my own reverse proxy
+        - Behind a tunnel (Cloudflare, Tailscale)
+        - Local development (pnpm dev)
+        - Not sure
+    validations:
+      required: true
+  - type: dropdown
+    id: surface
+    attributes:
+      label: Where did it happen
+      description: Which client were you using?
+      options:
+        - Web browser
+        - Desktop app (Electron)
+        - Installed PWA on mobile
+    validations:
+      required: true
+  - type: input
+    id: version
+    attributes:
+      label: Backspace version
+      description: The version shown in the app, or the image tag / commit you deployed.
+      placeholder: "1.0.0"
+    validations:
+      required: true
+  - type: dropdown
+    id: subsystems
+    attributes:
+      label: Does it involve voice or federation
+      description: These subsystems have their own moving parts, so it helps to know up front.
+      multiple: true
+      options:
+        - Voice or video (LiveKit)
+        - Screen sharing
+        - Federation between instances
+        - None of these
+  - type: textarea
+    id: logs
+    attributes:
+      label: Relevant logs
+      description: Server logs (docker compose logs backspace) or browser console output, if any. This is automatically formatted as code.
+      render: shell
+  - type: input
+    id: environment
+    attributes:
+      label: Browser and OS
+      placeholder: "Firefox 128 on Ubuntu 24.04, or Chrome 126 on Windows 11"

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Security vulnerability
+    url: https://github.com/TheZwiss/backspace/security/advisories/new
+    about: Do not open a public issue for security problems. Report privately here.
+  - name: Question or setup help
+    url: https://github.com/TheZwiss/backspace/discussions
+    about: For usage questions, deployment help, and general discussion.

.github/ISSUE_TEMPLATE/feature_request.yml

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+name: Feature request
+description: Suggest a new capability or an improvement to an existing one.
+labels: ["enhancement"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Open an issue before starting work on anything non-trivial, so we can agree on direction first.
+        Please check the README feature list and existing issues before filing.
+  - type: textarea
+    id: problem
+    attributes:
+      label: The problem
+      description: What are you trying to do that Backspace does not support today?
+      placeholder: As a space admin, I want to ... so that ...
+    validations:
+      required: true
+  - type: textarea
+    id: proposal
+    attributes:
+      label: Proposed solution
+      description: How you imagine it working. Rough ideas are fine.
+    validations:
+      required: true
+  - type: dropdown
+    id: area
+    attributes:
+      label: Area
+      description: Which part of the project does this touch?
+      options:
+        - Text chat and messaging
+        - Voice, video, or screen sharing
+        - Spaces, roles, and permissions
+        - Direct messages and friends
+        - Federation between instances
+        - Admin and moderation
+        - Desktop app
+        - Mobile and PWA
+        - Deployment and self-hosting
+        - Other
+    validations:
+      required: true
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives considered
+      description: Workarounds you have tried, or how other tools handle this.
+  - type: dropdown
+    id: federation-impact
+    attributes:
+      label: Federation compatibility
+      description: Would this need to work across peered instances? If unsure, leave the default.
+      options:
+        - Not sure
+        - Yes, it should work across federated instances
+        - No, it is local to a single instance

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thanks for contributing. Keep one logical change per pull request. -->
+
+## What this changes
+
+<!-- A short summary of the change and the problem it solves. Link the issue it addresses. -->
+
+Closes #
+
+## Type of change
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Refactor or cleanup
+- [ ] Documentation
+- [ ] Other
+
+## Checklist
+
+- [ ] `pnpm build` succeeds (shared types, server, and web all build)
+- [ ] `pnpm dev` starts the server and web client without errors
+- [ ] Tests pass where applicable (`pnpm test`)
+- [ ] I updated the relevant `docs/systems/` spec if this changed schema, API routes, WebSocket events, the federation protocol, permissions, or the design system
+- [ ] This change resolves the correct federated identity where it compares IDs, checks permissions, or talks to remote servers (no assumption of a single global user ID)
+- [ ] I have read and agree to the [CLA](../CLA.md)
+
+## Notes for reviewers
+
+<!-- Anything that helps review: screenshots for UI, migration notes, edge cases, follow-ups. -->