docs: add IndieAuth client identification analysis and decision

Architect analysis identified the root cause of 'client_id is not
registered' error: h-app microformat is hidden from parsers.

Includes:
- Complete diagnosis of IndieAuth client registration issue
- ADR-006: IndieAuth Client Identification decision record
- Implementation guidelines for developer

Developer task: Remove hidden attributes from h-app div.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docs/decisions/ADR-006-indieauth-client-identification.md

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+# ADR-006: IndieAuth Client Identification Strategy
+
+## Status
+Accepted
+
+## Context
+
+StarPunk needs to identify itself as an IndieAuth client when initiating authentication flows. The current implementation uses a hidden h-app microformat which causes IndieAuth services to reject the client_id with "This client_id is not registered" errors.
+
+IndieAuth specification requires clients to provide discoverable information about themselves using microformats. This allows authorization endpoints to:
+- Display client information to users
+- Verify the client is legitimate
+- Show what application is requesting access
+
+## Decision
+
+StarPunk will use **visible h-app microformats** in the footer of all pages to identify itself as an IndieAuth client.
+
+The h-app will include:
+- Application name (p-name)
+- Application URL (u-url)
+- Version number (p-version)
+- Optional: logo (u-logo)
+- Optional: description (p-summary)
+
+Implementation:
+```html
+<footer>
+  <div class="h-app">
+    <p>
+      Powered by <a href="https://starpunk.thesatelliteoflove.com" class="u-url p-name">StarPunk</a>
+      <span class="p-version">v0.6.1</span>
+    </p>
+  </div>
+</footer>
+```
+
+## Rationale
+
+1. **Specification Compliance**: IndieAuth spec requires client information to be discoverable via microformats parsing
+2. **Transparency**: Users should see what software they're using
+3. **Simplicity**: No JavaScript or complex rendering needed
+4. **Debugging**: Visible markup is easier to verify and debug
+5. **SEO Benefits**: Search engines can understand the application structure
+
+## Consequences
+
+### Positive
+- IndieAuth flows will work correctly
+- Client identification is transparent to users
+- Easier to debug authentication issues
+- Follows IndieWeb principles of visible metadata
+- Can be styled to match site design
+
+### Negative
+- Takes up visual space in the footer (minimal)
+- Cannot be completely hidden from view
+- Must be maintained on all pages that might be used as client_id
+
+## Alternatives Considered
+
+### 1. Hidden h-app with display:none
+**Rejected**: Some microformat parsers ignore display:none elements
+
+### 2. Off-screen positioning
+**Rejected**: Considered deceptive by some services, accessibility issues
+
+### 3. Separate client information endpoint
+**Rejected**: Adds complexity, not standard practice
+
+### 4. HTTP headers
+**Rejected**: Not part of IndieAuth specification, wouldn't work
+
+### 5. Meta tags
+**Rejected**: IndieAuth uses microformats, not meta tags
+
+## Implementation Guidelines
+
+1. **Placement**: Always in the footer, consistent across all pages
+2. **Styling**: Subtle but visible, matching site design
+3. **Content**: Minimum of name and URL, optional logo and description
+4. **Testing**: Verify with microformats parsers before deployment
+
+## Testing Checklist
+
+- [ ] h-app is visible in HTML source
+- [ ] No hidden, display:none, or visibility:hidden attributes
+- [ ] Validates at https://indiewebify.me/
+- [ ] Parses correctly at https://microformats.io/
+- [ ] IndieAuth flow works at https://indielogin.com/
+
+## References
+
+- [IndieAuth Spec Section 4.2.2](https://indieauth.spec.indieweb.org/#client-information-discovery)
+- [Microformats h-app](http://microformats.org/wiki/h-app)
+- [IndieWeb Client Information](https://indieweb.org/client-id)
+
+## Related ADRs
+
+- ADR-003: Authentication Strategy (establishes IndieAuth as auth method)
+- ADR-004: Frontend Architecture (defines template structure)