Phase 3: full share CRUD + session login page

Backend:
- SMB config parser/writer (_parse_smb_conf, _write_smb_conf) for
  safe whole-file rewrites instead of append-only
- NFS exports parser/writer (_parse_exports, _write_exports) with
  support for disabled (commented-out) exports
- DELETE/PATCH/toggle endpoints for SMB and NFS shares
- parse_smb() now returns enabled field (reads available = no)
- get_nfs_shares() uses the new parser and returns enabled field
- Replace HTTP Basic Auth with Flask session auth (secret key derived
  from MICRONAS_PASS so sessions survive restarts)
- /login GET/POST, /logout routes

Frontend:
- Login page (templates/login.html) with same dark theme
- Sign out button in header
- Edit modal pre-filled with current share values
- Delete buttons wired up (with confirm dialog)
- Enable/Disable toggle per share (green/grey dot status indicator)
- Disabled shares shown at reduced opacity

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/index.html

@@ -48,10 +48,30 @@
     </div>
 </div>
 
+<!-- Edit Share modal -->
+<div class="modal fade" id="editModal" tabindex="-1">
+    <div class="modal-dialog">
+        <div class="modal-content">
+            <div class="modal-header">
+                <h5 class="modal-title" id="editModalTitle">Edit Share</h5>
+                <button type="button" class="btn-close btn-close-white" data-bs-dismiss="modal"></button>
+            </div>
+            <div class="modal-body" id="editModalBody"></div>
+            <div class="modal-footer">
+                <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Cancel</button>
+                <button type="button" class="btn btn-primary" onclick="submitEdit()">Save</button>
+            </div>
+        </div>
+    </div>
+</div>
+
 <!-- Header -->
 <div class="topbar d-flex justify-content-between align-items-center">
     <span class="fw-semibold fs-5">MicroNAS</span>
-    <span id="lastRefresh" class="small" style="color:var(--muted)"></span>
+    <div class="d-flex align-items-center gap-3">
+        <span id="lastRefresh" class="small" style="color:var(--muted)"></span>
+        <a href="/logout" class="btn btn-sm" style="border:1px solid var(--border); color:var(--muted)">Sign out</a>
+    </div>
 </div>
 
 <div class="container-lg py-4">
@@ -147,7 +167,7 @@
             <div class="fw-semibold mb-2">Shares</div>
             <div class="table-responsive">
                 <table class="table table-striped mb-0">
-                    <thead><tr><th>Name</th><th>Path</th><th>Valid Users</th><th>Actions</th></tr></thead>
+                    <thead><tr><th>Name</th><th>Path</th><th>Valid Users</th><th></th><th>Actions</th></tr></thead>
                     <tbody id="smbTableBody">
                         <tr><td colspan="4" style="color:var(--muted)">Loading…</td></tr>
                     </tbody>
@@ -172,7 +192,7 @@
             <div class="fw-semibold mb-2">Exports</div>
             <div class="table-responsive">
                 <table class="table table-striped mb-0">
-                    <thead><tr><th>Path</th><th>Clients</th><th>Actions</th></tr></thead>
+                    <thead><tr><th>Path</th><th>Clients</th><th></th><th>Actions</th></tr></thead>
                     <tbody id="nfsTableBody">
                         <tr><td colspan="3" style="color:var(--muted)">Loading…</td></tr>
                     </tbody>