feat(timeline): start at today and allow infinite scroll into the past

Previously viewStart defaulted to today-30 and the scroll container had
no left-edge expansion logic, so users hit a hard wall when scrolling
left. This change:

- Sets viewStart default to today so the viewport opens with today at
  the left edge (URL ?startDate= override still respected).
- Adds left-edge auto-expansion in handleContainerScroll: when the user
  scrolls within 40 cells of the left boundary, 120 days are prepended
  and a useLayoutEffect applies the matching scrollLeft compensation in
  the same paint frame to prevent a visual jump.
- Floors backward navigation at 5 years (minDate) to prevent unbounded
  viewDays growth.
- Updates handleNavigateToday to match: resets to today rather than
  today-30.

Both resource view and project view use the same TimelineContext /
TimelineView, so both are fixed by this change.

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

apps/web/src/components/timeline/TimelineContext.tsx

@@ -284,7 +284,7 @@ export function TimelineProvider({
       const d = new Date(sp);
       if (!isNaN(d.getTime())) return d;
     }
-    return addDays(today, -30);
+    return today;
   });
   const [viewDays, setViewDays] = useState(() => {
     const sp = searchParams.get("days");
@@ -310,7 +310,7 @@ export function TimelineProvider({
         const d = new Date(spStart);
         if (!isNaN(d.getTime())) return d;
       }
-      return addDays(today, -30);
+      return today;
     });
 
     const spDays = searchParams.get("days");

apps/web/src/components/timeline/TimelineView.tsx

@@ -2,7 +2,7 @@
 
 import { clsx } from "clsx";
 import { useSession } from "next-auth/react";
-import { useCallback, useEffect, useMemo, useRef, useState } from "react";
+import { useCallback, useEffect, useLayoutEffect, useMemo, useRef, useState } from "react";
 import { useAllocationHistory } from "~/hooks/useAllocationHistory.js";
 import { useProjectDragContext } from "~/hooks/useProjectDragContext.js";
 import { useTimelineDrag } from "~/hooks/useTimelineDrag.js";
@@ -685,15 +685,31 @@ function TimelineViewContent({
   const scrollRafRef = useRef<number | null>(null);
   const [scrollLeft, setScrollLeft] = useState(0);
 
+  // Pixels to add to scrollLeft after a left-extension re-render (prevents jump).
+  const pendingLeftCompensationPx = useRef(0);
+
+  // Apply scroll compensation synchronously after the canvas grows leftward.
+  useLayoutEffect(() => {
+    const px = pendingLeftCompensationPx.current;
+    if (px === 0) return;
+    const el = scrollContainerRef.current;
+    if (el) el.scrollLeft += px;
+    pendingLeftCompensationPx.current = 0;
+  }, [viewStart]);
+
+  // 5-year floor — no practical data exists further back; prevents runaway growth.
+  const minDate = useMemo(() => addDays(today, -(365 * 5)), [today]);
+
   // ─── Navigation callbacks for TimelineToolbar ────────────────────────────
   const handleNavigateBack = useCallback(
-    () => setViewStart((v) => addDays(v, -28)),
-    [setViewStart],
-  );
-  const handleNavigateToday = useCallback(
-    () => setViewStart(addDays(today, -30)),
-    [setViewStart, today],
+    () =>
+      setViewStart((v) => {
+        const candidate = addDays(v, -28);
+        return candidate < minDate ? minDate : candidate;
+      }),
+    [setViewStart, minDate],
   );
+  const handleNavigateToday = useCallback(() => setViewStart(today), [setViewStart, today]);
   const handleNavigateForward = useCallback(
     () => setViewStart((v) => addDays(v, 28)),
     [setViewStart],
@@ -709,10 +725,31 @@ function TimelineViewContent({
   const handleContainerScroll = useCallback(() => {
     const el = scrollContainerRef.current;
     if (!el) return;
+
+    // Right-edge: extend future range
     const distanceFromRight = el.scrollWidth - el.scrollLeft - el.clientWidth;
     if (distanceFromRight < CELL_WIDTH * 40) {
       setViewDays((d) => d + 120);
     }
+
+    // Left-edge: prepend past range and compensate scroll position so viewport doesn't jump
+    if (el.scrollLeft < CELL_WIDTH * 40 && viewStart > minDate) {
+      const daysToPrepend = 120;
+      // Count the exact visible days (respecting showWeekends) being prepended
+      let prependedVisible = 0;
+      for (let i = 1; i <= daysToPrepend; i++) {
+        const d = addDays(viewStart, -i);
+        const dow = d.getDay();
+        if (filters.showWeekends || (dow !== 0 && dow !== 6)) prependedVisible++;
+      }
+      pendingLeftCompensationPx.current = prependedVisible * CELL_WIDTH;
+      setViewStart((v) => {
+        const candidate = addDays(v, -daysToPrepend);
+        return candidate < minDate ? minDate : candidate;
+      });
+      setViewDays((d) => d + daysToPrepend);
+    }
+
     scrollLeftRef.current = el.scrollLeft;
     if (scrollRafRef.current === null) {
       scrollRafRef.current = requestAnimationFrame(() => {
@@ -720,7 +757,7 @@ function TimelineViewContent({
         setScrollLeft(scrollLeftRef.current);
       });
     }
-  }, [CELL_WIDTH, setViewDays]);
+  }, [CELL_WIDTH, setViewDays, viewStart, minDate, setViewStart, filters.showWeekends]);
 
   // ─── Canvas mousemove — only forwards event when drag overlay is active ───
   const handleMouseMove = useCallback(