feat: add Share button and invite modal to document editor

internal/handler/handler.go

···
 	// AccessToken is the ATProto access token for WebSocket auth.
 	// Empty string if user has no ATProto session.
 	AccessToken string
+	// IsOwner is true when the current user owns (created) the document.
+	IsOwner bool
 	// IsCollaborator is true when the current user is in the collaborators list.
 	IsCollaborator bool
 }
···
 	doc.RKey = rkey
 
 	// Fetch ATProto session to pass access token to the WebSocket client.
-	editData := &DocumentEditData{Document: doc}
+	// The user is always the owner here since we fetched the doc via client.DID().
+	editData := &DocumentEditData{Document: doc, IsOwner: true}
 	if session, err := h.DB.GetATProtoSession(user.ID); err == nil && session != nil {
 		editData.AccessToken = session.AccessToken
 		// Check if this user is listed as a collaborator on the doc.
···
 	}
 	doc.RKey = rkey
 
-	session, err := h.DB.GetATProtoSession(user.ID)
-	if err != nil || session == nil {
-		http.Error(w, "Unauthorized", http.StatusForbidden)
-		return
-	}
-
-	parts := strings.Split(doc.URI, "/")
-	ownerDID := ""
-	if len(parts) >= 2 {
-		ownerDID = parts[1]
-	}
-	if ownerDID == "" || session.DID != ownerDID {
-		http.Error(w, "Unauthorized", http.StatusForbidden)
-		return
-	}
-
+	// The document was fetched via client.DID(), so the current user is always the owner.
 	if len(doc.Collaborators) >= 5 {
 		http.Error(w, "Maximum collaborators reached", http.StatusBadRequest)
 		return
 	}
 
-	invite, err := collaboration.CreateInvite(h.DB, rkey, session.DID)
+	invite, err := collaboration.CreateInvite(h.DB, rkey, client.DID())
 	if err != nil {
 		log.Printf("DocumentInvite: create invite: %v", err)
 		http.Error(w, "Failed to create invite", http.StatusInternalServerError)

static/css/editor.css

···
 .editor-page:has(~ .comment-sidebar) {
     right: 260px;
 }
+
+/* ── Collaboration: Invite modal ─────────────────────────────────────────── */
+
+.invite-modal {
+    position: fixed;
+    inset: 0;
+    background: rgba(0,0,0,0.4);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    z-index: 200;
+}
+
+.invite-modal-box {
+    background: var(--bg-card);
+    border: 1px solid var(--border);
+    border-radius: var(--radius);
+    box-shadow: 0 8px 32px rgba(0,0,0,0.2);
+    width: 440px;
+    max-width: calc(100vw - 2rem);
+}
+
+.invite-modal-header {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    padding: 0.75rem 1rem;
+    border-bottom: 1px solid var(--border);
+    font-weight: 600;
+    font-size: 0.9rem;
+}
+
+.invite-modal-close {
+    background: none;
+    border: none;
+    color: var(--text-muted);
+    cursor: pointer;
+    font-size: 1rem;
+    padding: 0.1rem 0.3rem;
+    line-height: 1;
+}
+
+.invite-modal-close:hover {
+    color: var(--text);
+}
+
+.invite-modal-body {
+    padding: 1rem;
+}

templates/document_edit.html

···
             {{if .IsCollaborator}}
             <div id="presence-list" class="presence-list" title="Active collaborators"></div>
             {{end}}
+            {{if .IsOwner}}
+            <button class="btn btn-sm btn-outline" id="btn-share" onclick="generateInvite()">Share</button>
+            {{end}}
             <button class="btn btn-sm btn-outline source-only active" id="btn-preview" onclick="togglePreview()">Preview</button>
             <button class="btn btn-sm btn-outline source-only" id="btn-wrap" onclick="toggleWrap()">Wrap</button>
             <button class="btn btn-sm btn-outline rich-only" id="btn-undo" onclick="richUndo()" title="Undo (⌘Z)">↩</button>
···
         </div>
     </div>
 </div>
+
+<!-- Invite modal -->
+{{if .IsOwner}}
+<div id="invite-modal" class="invite-modal" style="display:none">
+    <div class="invite-modal-box">
+        <div class="invite-modal-header">
+            <span>Share document</span>
+            <button class="invite-modal-close" onclick="closeInviteModal()">✕</button>
+        </div>
+        <div id="invite-modal-body" class="invite-modal-body">
+            <p>Generating invite link...</p>
+        </div>
+    </div>
+</div>
+{{end}}
 
 <!-- Comment sidebar -->
 {{if .IsCollaborator}}
···
         if (!linkTooltipEl.contains(e.target) && !document.getElementById('editor-rich').contains(e.target)) {
             hideLinkTooltip();
         }
+    });
+
+    // ── Invite ────────────────────────────────────────────────────────────────
+
+    async function generateInvite() {
+        const modal = document.getElementById('invite-modal');
+        const body = document.getElementById('invite-modal-body');
+        if (!modal) return;
+        body.innerHTML = '<p>Generating invite link...</p>';
+        modal.style.display = 'flex';
+
+        try {
+            const resp = await fetch(`/api/docs/${rkey}/invite`, { method: 'POST' });
+            const data = await resp.json();
+            if (!resp.ok) throw new Error(data.error || resp.statusText);
+            const link = data.invite_url || data.inviteLink || data.url || '';
+            body.innerHTML = `
+                <p style="margin:0 0 0.5rem;font-size:0.85rem;color:var(--text-muted)">
+                    Share this link. It expires in 7 days and can be used once.
+                </p>
+                <div style="display:flex;gap:0.5rem;align-items:center">
+                    <input id="invite-link-input" type="text" value="${escHtml(link)}" readonly
+                        style="flex:1;background:var(--bg);border:1px solid var(--border);border-radius:var(--radius);
+                               padding:0.4rem 0.5rem;font-size:0.85rem;color:var(--text);outline:none">
+                    <button class="btn btn-sm" onclick="copyInviteLink()">Copy</button>
+                </div>
+                <p id="invite-copy-msg" style="margin:0.4rem 0 0;font-size:0.8rem;color:var(--primary);display:none">Copied!</p>
+            `;
+        } catch (e) {
+            body.innerHTML = `<p style="color:var(--danger)">Failed to generate invite: ${escHtml(e.message)}</p>`;
+        }
+    }
+
+    function copyInviteLink() {
+        const input = document.getElementById('invite-link-input');
+        if (!input) return;
+        navigator.clipboard.writeText(input.value).then(() => {
+            const msg = document.getElementById('invite-copy-msg');
+            if (msg) { msg.style.display = 'block'; setTimeout(() => msg.style.display = 'none', 2000); }
+        });
+    }
+
+    function closeInviteModal() {
+        const modal = document.getElementById('invite-modal');
+        if (modal) modal.style.display = 'none';
+    }
+
+    // Close invite modal on backdrop click
+    document.getElementById('invite-modal')?.addEventListener('click', e => {
+        if (e.target === document.getElementById('invite-modal')) closeInviteModal();
     });
 
     // ── WebSocket / Collaboration ─────────────────────────────────────────────