Add plan dates, postponing, backlog and projects

- Tasks get an optional plan date separate from the deadline. Unfinished
  plans roll over in Today; only missed deadlines count as overdue.
- Postpone sheet on every task (tomorrow, weekend, next week, ...) that
  can move either the plan date or the deadline, with undo.
- Backlog view for undated tasks, with suggestions in Today.
- Projects with deadline, notes, progress, next step and "falling
  behind" warnings, shown in Today and in their own view.
- Times and the calendar now follow the plan date (else the deadline).
- Schema v3 migration (backup taken first); export format v3 includes
  projects and still imports older files.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/static/style.css

@@ -104,7 +104,10 @@ textarea { resize: vertical; }
 }
 .brand { display: flex; align-items: center; gap: 10px; font-weight: 700; font-size: 18px; padding: 4px 10px 16px; }
 .nav { display: flex; flex-direction: column; gap: 2px; }
-.nav-heading { font-size: 12px; text-transform: uppercase; letter-spacing: 0.06em; color: var(--muted); padding: 18px 10px 6px; }
+.nav-heading {
+  display: flex; align-items: center; justify-content: space-between;
+  font-size: 12px; text-transform: uppercase; letter-spacing: 0.06em; color: var(--muted); padding: 18px 10px 6px;
+}
 .nav-item {
   display: flex; align-items: center; gap: 10px; width: 100%; text-align: left;
   background: none; border: none; border-radius: 8px; padding: 8px 10px; cursor: pointer;
@@ -300,3 +303,77 @@ input:disabled { opacity: 0.5; }
   .week-strip { gap: 4px; }
 }
 .block.short .block-time { order: 2; }
+
+/* Small helpers */
+.small { font-size: 12px; }
+.small-label { font-size: 13px; align-self: center; }
+.icon-btn.small { padding: 2px 6px; font-size: 15px; line-height: 1; }
+
+/* Sidebar: projects */
+.nav-item.muted { color: var(--muted); }
+.nav-toggle {
+  background: none; border: none; color: var(--muted); font-size: 12px; cursor: pointer;
+  text-align: left; padding: 4px 10px 4px 40px;
+}
+.nav-toggle:hover { color: var(--text); }
+.nav-empty { color: var(--muted); font-size: 13px; padding: 2px 10px 2px 40px; }
+
+/* Task row: plan chip and postpone button */
+.chip.plan { color: var(--accent); }
+.chip.plan.now { background: var(--accent-weak); font-weight: 600; }
+.link-chip { cursor: pointer; }
+.link-chip:hover { color: var(--accent); }
+.plan-btn { align-self: center; font-size: 16px; }
+.group-hint { margin: -4px 0 8px; font-size: 13px; color: var(--muted); }
+
+/* Projects */
+.progress { flex: 1; height: 8px; background: var(--panel-2); border-radius: 999px; overflow: hidden; }
+.progress-fill { height: 100%; background: var(--ok); border-radius: 999px; }
+.project-progress { display: flex; align-items: center; gap: 10px; margin-top: 8px; }
+.project-progress.big .progress { height: 10px; }
+.project-card {
+  background: var(--panel); border: 1px solid var(--border); border-left: 4px solid var(--accent);
+  border-radius: var(--radius); padding: 11px 14px; margin-bottom: 8px; cursor: pointer; box-shadow: var(--shadow);
+}
+.project-card:hover { border-color: var(--accent); }
+.project-card-head { display: flex; justify-content: space-between; align-items: baseline; gap: 10px; flex-wrap: wrap; }
+.project-name { font-weight: 600; }
+.project-next { margin-top: 6px; font-size: 14px; overflow-wrap: anywhere; }
+.project-warn { margin-top: 6px; font-size: 13px; color: var(--today); font-weight: 600; }
+.project-header {
+  background: var(--panel); border: 1px solid var(--border); border-radius: var(--radius);
+  padding: 14px; margin-top: 6px; box-shadow: var(--shadow);
+}
+.project-header-top { display: flex; justify-content: space-between; align-items: center; gap: 10px; flex-wrap: wrap; }
+.project-header-top .task-meta { margin-top: 0; }
+.project-notes { white-space: pre-wrap; margin: 10px 0 0; color: var(--muted); overflow-wrap: anywhere; }
+.check-field { display: flex; align-items: center; gap: 8px; cursor: pointer; }
+.deadline-line {
+  display: block; width: 100%; text-align: left; background: var(--panel); border: 1px dashed var(--border);
+  border-radius: var(--radius); padding: 10px 12px; margin-bottom: 8px; cursor: pointer;
+}
+
+/* Postpone sheet */
+.segmented { display: flex; background: var(--panel-2); border-radius: 9px; padding: 3px; gap: 3px; }
+.segmented button {
+  flex: 1; border: none; background: none; border-radius: 7px; padding: 7px; cursor: pointer; color: var(--muted);
+}
+.segmented button.active { background: var(--panel); color: var(--text); font-weight: 600; box-shadow: var(--shadow); }
+#p-current { margin: 0; }
+.plan-options { display: grid; grid-template-columns: 1fr 1fr; gap: 8px; }
+.plan-option {
+  display: flex; flex-direction: column; align-items: flex-start; gap: 2px; text-align: left;
+  background: var(--panel); border: 1px solid var(--border); border-radius: 10px; padding: 10px 12px; cursor: pointer;
+}
+.plan-option:hover { border-color: var(--accent); background: var(--accent-weak); }
+dialog.sheet { width: min(420px, calc(100vw - 24px)); }
+
+@media (max-width: 800px) {
+  /* The postpone sheet slides up from the bottom on phones. */
+  dialog.sheet {
+    width: 100vw; max-width: 100vw; margin: auto 0 0; border-radius: 16px 16px 0 0;
+    padding-bottom: env(safe-area-inset-bottom);
+  }
+}
+#plan-dialog .row { flex-wrap: nowrap; }
+#plan-dialog .row input { flex: 1; }