Add minimal web UI

plans/PHASE_5_WEB_UI_PLAN.md

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+# Phase 5 Web UI Plan
+
+Goal: add a very small server-rendered web UI with minimal styling while preserving CLI/API behavior.
+
+## Scope
+
+- [x] `/repos` repository search/list page
+- [x] `/login` login form with cookie auth
+- [x] `/register` registration form
+- [x] `/logout` form action
+- [x] `/repos/new` repository creation form
+- [x] `/{username}/{repo}` repository overview and root file tree
+- [x] `/{username}/{repo}/tree` file tree browsing
+- [x] `/{username}/{repo}/blob` file viewing
+- [x] `/{username}/{repo}/edit` simple textarea editor for files
+- [x] `/{username}/{repo}/fork` public-repository-only fork action
+- [x] Minimal CSRF protection for cookie-backed POST forms
+- [x] Tests for routing, auth forms, repository pages, and permission-sensitive actions
+
+## Non-goals
+
+- No JavaScript framework
+- No rich CSS/theme system
+- No SSH
+- No CI/CD
+- No advanced code review UI
+
+## Completion
+
+- [x] Implemented
+- [x] Tests pass