Per-pane and global terminal zoom via keyboard

Each leaf now carries an optional fontSizeOffset, persisted in
workspace.json alongside everything else. Ctrl+= / Ctrl+- / Ctrl+0
adjust the active pane; adding Shift escalates to every pane (the
mirror of the broadcast Shift+Alt convention, with shift alone since
the keys are otherwise unused). Bindings match on e.code so layouts
that don't have "=" / "-" / "0" in the same spot still work.

XtermPane gained a fontSize prop. A secondary effect reacts to changes:
set term.options.fontSize, fit() to recompute cols/rows for the new
cell size, refresh(), then resizePane so bash redraws the prompt at
the right width. No remount, so PTY + scrollback survive zoom changes.

The new tree helpers (resolveFontSize / adjustFontSize /
adjustAllFontSizes) are metadata-only — they don't swap leaf ids, so
nothing respawns. reshapeToPreset also carries the offset across when
splicing existing leaves into a new layout. 12 new vitest cases pin
those invariants plus the clamp and reset-to-default behaviour.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/layout/tree.ts

@@ -25,8 +25,21 @@ export interface LeafNode {
    * pane toolbar.
    */
   broadcast?: boolean;
+  /**
+   * Per-pane font-size delta from the default ({@link DEFAULT_FONT_SIZE}).
+   * Bumped by Ctrl+Shift+= / Ctrl+Shift+- / reset by Ctrl+Shift+0.
+   * Stored as an offset (not absolute) so changing the base default
+   * later doesn't require migrating saved workspaces.
+   */
+  fontSizeOffset?: number;
 }
 
+/** Base xterm.js font size in px. Per-leaf offset adds on top of this. */
+export const DEFAULT_FONT_SIZE = 13;
+/** Hard clamps on `DEFAULT_FONT_SIZE + offset`. */
+export const MIN_FONT_SIZE = 6;
+export const MAX_FONT_SIZE = 40;
+
 export interface SplitNode {
   kind: "split";
   id: NodeId;
@@ -198,6 +211,59 @@ export function toggleBroadcast(root: TreeNode, leafId: NodeId): TreeNode {
   });
 }
 
+/** Compute the actual pixel font size from a leaf's offset, clamped to
+ *  [MIN_FONT_SIZE, MAX_FONT_SIZE]. */
+export function resolveFontSize(offset: number | undefined): number {
+  const px = DEFAULT_FONT_SIZE + (offset ?? 0);
+  return Math.max(MIN_FONT_SIZE, Math.min(MAX_FONT_SIZE, px));
+}
+
+/** Apply a font-size change to one leaf. Internal helper; returns the
+ *  same reference when nothing changes so callers can short-circuit. */
+function adjustOneFontSize(leaf: LeafNode, delta: number | null): LeafNode {
+  if (delta === null) {
+    if (leaf.fontSizeOffset === undefined) return leaf;
+    const next: LeafNode = { ...leaf };
+    delete next.fontSizeOffset;
+    return next;
+  }
+  const cur = leaf.fontSizeOffset ?? 0;
+  const nextPx = resolveFontSize(cur + delta);
+  const nextOffset = nextPx - DEFAULT_FONT_SIZE;
+  if (nextOffset === cur) return leaf;
+  if (nextOffset === 0) {
+    const next: LeafNode = { ...leaf };
+    delete next.fontSizeOffset;
+    return next;
+  }
+  return { ...leaf, fontSizeOffset: nextOffset };
+}
+
+/** Adjust a single leaf's font-size offset by `delta` (positive = bigger).
+ *  Pass `delta = null` to reset back to the default. Metadata-only — does
+ *  NOT swap the id, so the PTY keeps running. */
+export function adjustFontSize(
+  root: TreeNode,
+  leafId: NodeId,
+  delta: number | null,
+): TreeNode {
+  return replaceById(root, leafId, (node) => {
+    if (node.kind !== "leaf") return node;
+    return adjustOneFontSize(node, delta);
+  });
+}
+
+/** Adjust EVERY leaf's font-size offset by the same `delta` (or reset all
+ *  to default with `delta = null`). Independent per-pane offsets stay
+ *  independent — we just shift each by the same amount. */
+export function adjustAllFontSizes(root: TreeNode, delta: number | null): TreeNode {
+  if (root.kind === "leaf") return adjustOneFontSize(root, delta);
+  const a = adjustAllFontSizes(root.a, delta);
+  const b = adjustAllFontSizes(root.b, delta);
+  if (a === root.a && b === root.b) return root;
+  return { ...root, a, b };
+}
+
 /**
  * Reshape the tree into the structure produced by `preset`, but PRESERVE
  * existing leaves (and their PTYs) by copying their id/distro/cwd/label/
@@ -231,6 +297,7 @@ export function reshapeToPreset(
     if (src.cwd !== undefined) slot.cwd = src.cwd;
     if (src.label !== undefined) slot.label = src.label;
     if (src.broadcast !== undefined) slot.broadcast = src.broadcast;
+    if (src.fontSizeOffset !== undefined) slot.fontSizeOffset = src.fontSizeOffset;
   }
 
   for (let i = slots.length; i < existingLeaves.length; i++) {