fix: compress linear tree chains to prevent JSON.stringify stack overflow

[fallleave001]

Problem

On long linear sessions (2000+ messages), JSON.stringify(tree) in the session detail API throws Maximum call stack size exceeded because each message is a single-child node forming a deep linear chain.

Simply removing the tree field breaks BranchNavigator branch navigation.

Solution

Add a generic compressTree function that contracts linear single-child chains while preserving branch points and terminal leaves:

• Linear chains (node has exactly 1 child): skip intermediate nodes iteratively (while loop, no stack growth) and connect the chain head directly to the tail.
• Branch points (node has 2+ children): preserve the node and recurse on each child.
• Leaves (node has 0 children): return as-is.

Before: Msg_1 → Msg_2 → ... → Msg_3400 (depth 3400 → stack overflow)
After: Msg_1 → Msg_3400 (depth 2 → safe)

Branch tree shape preserved:

  root → A → [branch1 → leaf1, branch2 → leaf2]
  root → [branch1 → leaf1, branch2 → leaf2]   ✅ leaves preserved

Testing

• Tested with real 3500+ message session containing 5 branch points
• BranchNavigator navigation works correctly
• activeLeafId resolves to existing nodes

Notes

Supersedes PR #38 and PR #48 following reviewer feedback:

• v1: remove tree → rejected (breaks BranchNavigator)
• v2: recursive compress → reviewer noted leaf nodes lost
• v3: iterative chain descent + leaf preservation → current