Add optional time ranges and a calendar day view

Tasks can now have a start/end time on their deadline date. The new
Calendar view shows a day as a timeline (with a week strip and projected
repeats of recurring tasks), and the task form lists what is already
planned that day and flags overlaps. Existing databases are migrated to
schema v2 automatically, with a backup copy taken first.

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

app/static/index.html

@@ -47,6 +47,12 @@
         <button class="btn primary" type="submit">Add</button>
         <button id="new-detailed" class="btn" type="button" title="New task with details">＋ Details</button>
       </form>
+      <div id="cal-nav" class="cal-nav" hidden>
+        <button id="cal-prev" class="btn" type="button" aria-label="Previous day">‹</button>
+        <input id="cal-date" type="date" aria-label="Day">
+        <button id="cal-next" class="btn" type="button" aria-label="Next day">›</button>
+        <button id="cal-today" class="btn" type="button">Today</button>
+      </div>
       <div id="list" class="list"></div>
     </main>
   </div>
@@ -99,6 +105,17 @@
         <button type="button" class="chip-btn" data-days="7">Next week</button>
         <button type="button" class="chip-btn" data-days="">No date</button>
       </div>
+      <div class="row">
+        <label class="field">
+          <span>From <small>(optional)</small></span>
+          <input id="t-start" type="time" step="300">
+        </label>
+        <label class="field">
+          <span>To</span>
+          <input id="t-end" type="time" step="300">
+        </label>
+      </div>
+      <div id="t-day" class="day-preview" hidden></div>
       <label class="field">
         <span>Tags <small>(comma separated)</small></span>
         <input id="t-tags" placeholder="e.g. editing, urgent" autocomplete="off">