feat: add sync detection for repositories (#28)

Reviewed-on: #28

docs/sync-detection-plan.md

@@ -0,0 +1,93 @@
+# Nostr sync detection plan
+
+Detect when a local repository has diverged from its NIP-34 state on Nostr
+(kind 30618) and suggest syncing when the app opens.
+
+Scope: repositories already initialized on Nostr (NIP-34 binding present).
+Purely local repositories without a binding do not get this feature.
+
+Scenario: the repository is initialized on Nostr and also pushed to Gitea.
+Work continues locally and is pushed to Gitea only, so the Nostr state falls
+behind. Signed should notice the difference on startup and offer to sync.
+
+## Prior art
+
+- nak (`git.go`): `nak git sync` fetches the latest kind 30617/30618 events
+  from relays and writes them to local refs (`refs/nip34/state/*`); push and
+  pull then compare against those refs with ordinary git machinery
+  (e.g. `merge-base --is-ancestor` for fast-forward checks).
+- ngit-cli: the same idea — the Nostr state is materialized as git refs in the
+  repository, then ahead/behind comparison applies.
+
+## Current state of the codebase
+
+| Piece | Location | Status |
+|---|---|---|
+| NIP-34 binding detection | `Repo::nip34_binding()` in `signed_git/src/repo.rs` | done |
+| Full Nostr state (refs + HEAD) | `RepoState::parse` in `signed_core/src/state.rs` | parsed, but `RepoStore` keeps only `head` and drops the refs |
+| Push-to-Nostr flow | `Backend::push_repo_from` / `RepoStore::push_checkout` / `push_repository` | done; stages the state event on grasp relays, pushes objects, broadcasts 30618 |
+| "Ahead of origin" detection | `CheckoutsStore::checkout_push_status` — local branch vs `refs/remotes/origin/<branch>` | exists, but that is local vs Gitea, not local vs Nostr |
+| UI suggestion surfaces | Sidebar unpushed badge (`SidebarPanel::refresh_unpushed`), push banner (`RepoDetailView::push_suggestion` / `render_push_banner`) | pattern exists for origin; nothing for Nostr |
+
+Gap: nothing compares the local repository's refs against the Nostr
+`RepoState` refs, and no "sync to Nostr" suggestion is based on that
+comparison.
+
+## Plan
+
+### Phase 1 — Divergence computation in `signed_git`
+
+Add a pure function in a new `sync.rs` module that takes local `ref_state()`
+and the Nostr `RepoState.refs` and classifies each `refs/heads/*` ref:
+
+```rust
+pub enum RefSync {
+    InSync,
+    LocalAhead { ahead: usize },            // fast-forwardable push
+    RemoteAhead { behind: usize },          // Nostr has newer commits
+    Diverged { ahead: usize, behind: usize },
+    LocalOnly,   // branch never published
+    RemoteOnly,  // branch exists only on Nostr
+}
+
+pub struct RepoSyncStatus {
+    pub refs: Vec<(String, RefSync)>,  // branch name -> classification
+    pub ahead_total: usize,            // summary for badges
+    pub behind_total: usize,
+}
+```
+
+Implemented with the existing `Repo::merge_base` and `commits_since` /
+`commits_ahead` — no new dependencies.
+
+### Phase 2 — Keep Nostr state refs and add a sync-status store in `signed_state`
+
+1. `RepoStore`: store `state_refs: Vec<(String, String)>` (and the state event
+   timestamp) alongside `head` in `run_refresh` instead of discarding them.
+2. New `SyncStatusStore` (following the `CheckoutsStore` pattern: global
+   entity, background compute, notify-on-change) that:
+   - watches own announcements (`RepoListStore` + `Backend::current_user`;
+     for repositories the user does not own, sync means PRs, which is out of
+     scope);
+   - reads the latest kind 30618 event per own repository from the Nostr LMDB
+     (`RepoListStore::sync_own_repo_states` already ensures those events are
+     synced);
+   - for each associated checkout (`CheckoutsStore::associations_of`)
+     computes `RepoSyncStatus` against the local refs on the background
+     executor;
+   - recomputes on app open, `BackendEvent::Synced` / `RepoUpdates` (state
+     kind), after a successful push, and on a slow poll (~60s, like
+     `PUSH_POLL`).
+
+### Phase 3 — UI: suggest sync
+
+- Sidebar: out-of-sync badge on own repository rows, reusing the `unpushed`
+  badge pattern, visually distinct (e.g. "N commits not on Nostr" tooltip).
+- RepoDetailView: banner modeled on `render_push_banner`: "main is 3 commits
+  ahead of Nostr — Sync now", calling the existing `store.push_checkout(path)`.
+
+## Open decisions
+
+1. Badge placement: sidebar only, or sidebar plus repo-detail banner in the
+   first pass. Recommendation: both — the banner hosts the action button, the
+   badge drives the click.